They’re similar to tilesets in that there are 256 of them, each for every ASCII character. None of the game engines I know of support PM’s font format (which is pretty clever, I admit), they’re either have support for bitmap fonts that are monospace (usually a 16x16 grid of characters of constant size, each representing one ASCII value) or BMFont for bitmap fonts that are variable-width. I wouldn’t even be making fuss over it, if not for the fact that PM can display what character you’re supposed to draw in a specific cell, which saves you from headache of having to fuss with ASCII tables and whatnot, which is present only when choosing bitmap font as a format.
This is on my list of feature wishes. At the moment I think of just having an importer/exporter for the grid based mono space format. BMFont is somewhat complex with dozens of settings. But also with this it should be enough to have just file i/o. The most important thing is that BMfont supports non ASCII characters which is what the Pro Motion format misses. But this could be done by extending the PM bitmap font creation engine to let the user define the characters himself. That means you just tell the system the character order (now fixed to ASCII).
This way you can eliminate all characters that are not required and give only those that you need.
You set up “ABCDEF” and the first characters in your font are just ABCDEF or cześć or 你好. The order definition is then part of the font definition.
For coupling with game engines the file i/o for the (mono space, bmfont) formats are used then. Maybe also some important extra settings can be made available within the project settings for bitmap fonts.
Hope this makes sense 
Yup, that makes sense to make it an export option. And I didn’t realize the character map was hardcoded.
Merry Christmas, by the way!