Riivolution can replace any file in any game. So any game can be "hacked" (a more correct term would be "modified", really).
Of course, many games use proprietary formats and whatnot, meaning to actually make working replacements you'd probably have to reverse engineer the formats used. Then again, a lot of games will use more "standard" formats from the Wii SDK, many of which have already been reverse engineered and understood (see WiiBrew wiki), and many tools written that can handle them (again, WiiBrew wiki has quite a few, others will be scattered across the internet in various forum posts and wiki pages). A quick google search for the format in question can often turn up some useful information and/or tools.
Look here:
http://www.mariokartwii.com/f101/my-ultimate-tutorial-resource-119338-2.html#post5624670
I haven't actually tried this, but from the looks of things, it would
appear you just need to convert two images with all the characters to TPL and paste it into the BRFNT file with a hex editor. Although I'm not sure if there are any open source tools which allow converting two images into one TPL or not (instead of using TexConv, which is from the SDK and therefore illegal to distribute).
EDIT: This might be a better guide:
http://www.mariokartwii.com/f72/how-create-your-own-mkw-font-nutshell-81870.html