Can font substitutions in MS Word be controlled?

Suppose that I am typing text in MS Word (any version) and I enter a character that does not exist in the font being used. Say, I’m using Times New Roman and I type 2300 Alt X, which turns to the diameter sign “⌀”, which does not exist in Times New Roman. MS Word picks it up from a different font, like Arial Unicode MS. This may mess up the typographic style, or line spacing. And this happens without notice.

Perhaps the most inconvenient feature here is that MS Word does not automatically return to the original font. Subsequent text appears in the replacement font, unless the user sees what is happening and realizes that he needs to change the font.

The question is: Can such substitutions be controlled, e.g. by specifying the font(s) to be used as backup fonts? If not, is there any reliable documentation about it?

You can record a macro to get around the problem. You need the macro to do the following:[space] [left-arrow] Alt-X [right-arrow]

To use it, type 2300 immediately followed by the key you've assigned to the macro. The macro inserts a space character (which will be of the orginal font), goes back to your number, does the Alt-X to convert it to unicode, then goes back to after the space (which is again in your original font). If you don't want to insert a space (if you want the next character to be immediately after the unicode character), add a [backspace] at the end of your macro.