Good thinking, that man!

Solution 1:

It's Terry Pratchett, so I think we can safely say the primary "meaning" is a humorous variation on sidekick Robin's constantly-repeated adulatory...

"Good thinking, Batman!" (as "parrotted" by myself and playmates 50 years ago)

Thus it's no more a "regionalism" than saying some equivalent like "[That's a] good idea, John!" - it's usually just a witty allusion to a 60s catchphrase. But the words themselves are just normal English, so obviously they were/are/will be used without that allusion sometimes.

But it's not common. There are just 12 instances in Google Books, of which I can only read the text in 8 cases (half of which are from Pratchett's own books). If you do hear it "out in the wild", you might get a clue from the enunciation as to whether the speaker himself is aware of the Pratchett allusion by noting whether thatman is "slurred together" into a single word.

As regards "common idiomatic usages", it's worth noting that forms such as "Well done, that man!" and "Well said, that man!" are quite natural (there are over a thousand written instances of each, in those links). But whereas it's perfectly normal to use the Batman version (or the jocular allusion thereto) when there's only the "addressee" present, it's unlikely you'd use that man in any other context, unless there were other people present at the time (the ones you're really addressing).

EDIT: Noting comments below (and the fact that a couple of people disagree with what I'm saying so strongly as to downvote it), I think it's worth considering the implications of this NGram...

It's a reasonable assumption that some of the capitalised instances will arise in contexts such as "Good thinking is thinking that is properly guided by epistemological criteria", and some of the non-capitalised instances will be things like "That was good thinking, son", for example. But it's difficult to ignore two implications:-

a) This type of "gerund" usage was far less common in C19 than C20 (so it didn't "tail off" after WW1)
b) The capitalised (possibly satirical) "plaudit" version only really took off after the Batman TV shows.

Solution 2:

The expression is rooted in the English class system, where a member of the upper classes would not necessarily remember (or even know, or care about) the name of someone from a lower class.

In the exclamation, "Good thinking, that man!" the speaker is exclaiming to the world in general that a man had had a good idea. He can't bring himself to talk to the man himself, because that would mean having to address him by name — even if just the surname, "Good thinking, Jones!", a name which he doesn't know. So instead, he announces appreciation to his peers, calling the underling "that man", "that man over there", and so avoids having to address him at all.

So yes: it is a real English saying, if rather dated. It wouldn't have been used in this way since the class system largely collapsed in the First World War.

It's become a caricatured trait, and these days someone might say it in appreciation instead of saying "Good idea, Bob" to his friend standing next to him.

Solution 3:

I found nothing to suggest that the phrase Good thinking, that man existed prior the 1980s and hence Andrew Leach's seductive opening statement that this form of utterance harks back to the British Empire era and its class system, although very appealing appears to be, at best, misguided.

The expression is rooted in the English class system, where a member of the upper classes would not necessarily remember (or even know, or care about) the name of someone from a lower class.

The following phrases while they do acknowledge that the construction Good thinking + old + noun predate and are not exclusive to Pratchett; they do not, surprisingly, predate the 1950s which frankly I had not expected.

Good thinking, old man. Trusts and Estates, Volume 96, New York City Fiduciary Publishers, 1957

Good thinking, old twin. A Trick of Light by Barbara Corcoran, 1972 (USA)

Good thinking, old buddy. Star Smashed of the Galaxy Rangers by Harry Harrison, 1974 (USA)

Good thinking, old girl Sights and Sounds, Scents and Savors of Maine by Harvey and Jean Howells, 1978 (UK)

Good thinking, old fellow. The Plains of the Sea by Niel Hancock, 1982 (USA)

Jolly good thinking, old chap. Kith and Kin by André Kaminski, 1988 (USA)

Good thinking, old thing The Doggone Years by Masood Hasan, 1996 (Pakistan)

Good thinking, old bean Royal Blood by Rhys Bowen 2010 (set in London, England, 1932; a story about a member of the Royal family who attends a wedding in Transylvania.)

The earliest instance of "Good thinking, that man" that I found on the Internet is dated February 1, 2001, not conclusive proof I agree but it is an indication that this expression is not some relic of the British upper classes but, au contraire, a well-oiled American English construction which is alive and kicking today.

If there was one English author who I would have thought used this expression in his body of work, it would be P.G. Wodehouse as Wikipedia points out

Wodehouse's main canvas remained that of a pre- and post-World War I English upper class society, reflecting his birth, education and youthful writing career.

The phrases "Good thinking, Jeeves" or any of their derivatives (e.g. "Good thinking, old man") simply do not exist in any of Wodehouse's novels.

Finally, @Fumblefingers assertion that the phrase is a derivation of Good thinking, Batman is legitimate and confirmed in A Dictionary of Catch Phrases By Eric Partridge. The famed etymologist states that the catchphrase Good thinking! didn't become a common British English expression until circa 1969 and...

The phrase came from the US, where it had arisen, c. 1950, among the advertising and publicity agencies of New York's madison Avenue and had, by the middle or late 1950s, become a US c.p.—not unassisted by the "Good thinking, Batman" of the Batman 'comic' strips.