JoeQuake/FuhQuake/ezQuake/Qrack engines (very similar codebase in many ways, with FuhQuake being the origin and the others being the forks) have a loadcharset command.
Hence, those engines have a wide variety of character sets available.
One example:
http://www.moondrunk.se/gfx_charsets.php
Anyway, if you want to load non-128x128 charsets, look to the JoeQuake source code.
You have 2 choices on how to handle it:
1. Change the engine to use, say, 512 x 512 for .lmp loading and alter all graphics functions drawing characters to use 32x32 for the character size instead of 8x8.
2. Probably easier, add a "layer" on top where a replacement element is accepted and draw that instead if found. (This is what JoeQuake and similar engines do).
By the way ...
WARNING ... you guys are thinking this is "easy" but think about how many different things get changed all over the place especially if you do option #1, you'll have drawing changes everywhere from things like the console display to center printing to the status bar and the notify text and on and on ... plus depending on how you do it, it will change the number of rows and columns that can be displayed. And if you are modifying an engine, like a PSP one, there are several factor that will make this a major headache (doubly so on the PSP, due to screen size of 480 x 272).
With DarkPlaces, you can put a c:\(your Quake folder)\id1\gfx\conchars.tga replacement element of any size (like those ones on MoonDrunk's page.)