Well if only ttf fonts can be used then the best thing to do is use one that does not have extra glyphs in it. Don't use a font that has all the extra character for EU and other languages, or diacritics or even things like â„¢ and junk. Keeping it to just the commonly-used characters for US English can bring down the workload on the renderer a bit. There's lots of fonts like this, but the only ones I've seen that were also readable were some bold versions (even the default font is bolded) of sans-serif fonts. dafont.com, IIRC, has marks on the font list for the ones that support extra characters or not and tends to package all the versions into one zip so you can just find a barebones font and use the bold version.
However we know that actual bitmap-format fonts are usable since Abyss patches one in (a bunch of older memory patchers did too), the question is how.