Hardly/Scarcely...than

AHD says about the use of the adverb hardly:

Hardly and other minimizing adverbs are properly followed by when and not than in sentences like

• I had hardly walked inside [when/than] it began to rain.

In our 2008 survey, 79 percent of the Usage Panel rejected the use of than in the previous sentence.

So the use with than is still controversial and the main argument against it is that it normally follows comparatives (like sooner in no sooner... than). However others have argued that in hardly... than, than is used to show contrast between two situations.

To my surprise, I find than acceptable (though not as good as when) in

• Hardly had he come home than it started to rain.

It's terrible without the negative preposing, though:

• *He had hardly come home than it started to rain

which makes me suspect this is a phenomenon of negation.

After all, aside from the fact that hardly, barely, scarcely, and rarely are all negative triggers themselves, baseline comparison clauses in comparatives (the clauses that begin with than) are also negative environments, as the NPI ever below shows.

• He got wetter than he ever expected on the walk home.

EDIT

In retrospect, I think

• Hardly had he come home than it started to rain.

is acceptable because hardly is a mimic of the common idiom no sooner

• No sooner had he come home than it started to rain.

which is both negative and comparative, therefore licensing than. Hardly is somewhat negative, as noted, but not comparative; no sooner is both.

To the extent that hardly had he sounds like no sooner had he, it works.
Of course, mimicry is very much a YMMV phenomenon.