After dealing with GameMaker Studio's janky kerning when rendering text, I'm very much up for this. A font preview might be perfect, but the rendered text on screen sometimes randomly kerns and sometimes doesn't. I think it depends on the phase of the moon and random orbital decay of atoms around the Yo Yo offices. I need a better solution. 

Any chance that the inline style codes will allow for colour changes as well? I've got tooltip boxes that would really benefit from that and I've not got around to coding that yet (in the 'too hard, leave it for now' bucket).

Well, I hope this will fit your needs!  Right now it's in beta.  Still need to write up documentation and I'm still doing internal testing... but that also means I'm a bit more open to feature requests.  You can adjust kerning either in line or using a pre-built style.  It would probably be annoying to do it like... for a font, though.  Might be best to use a sprite font if the whole font needs to adjust for specific characters.  You get a bit more control that way.  In fact, if you use a sprite font and then over tune the kerning on all characters except for the ones that have issues, you may be able to use this to adjust kerning wherever you are using this font in a single style.