I don't have too much insight in how Textmate's plug-ins nor bundles work… but after I fiddled around with some of the preferences and files, I soon came to a conclusion that It's hard to change the font-preference depending on what file-type it is.
I would like to use a custom font JUST when viewing a .nfo file.
Setting a custom theme, font, etc for a specific language isn't possible. Don't remember font actually being requested before, theme- per-language is a very common (known) request though.