Hi duo_decimus! 
I think that the best option would be using a known and tested bitmap font format, maybe the one used by BMFont: BMFont - AngelCode.com
This format consists in a .png file (you can configure this on “option>export options>texture>png”) plus a plain txt file (the extension is .fnt, but it’s a txt anyway).
That way people could directly use the fonts that they already have, and since the format has been quite tested through the years, you’d also avoid the pitfalls you were worried about ^^
On the other hand, while this is the most complete option, it’s probably a bit less user-friendly in terms of creating the fonts than, for example, idea #4.
Thank you for your work! and please let us know if we can help! 