Speaking only in theory, it shouldn't be that hard to swipe tiles from Dwarf Fortress, assuming it is widely known how the official GFX mode is looking to load the images (or textures). As long as that information is available it should be easy to make custom tiles, or at least write something to convert an image into a "tile", possibly splicing it into a data file.
As for the processing concerns, I can't see why it would peg the processor. The OpenGL term I wrote barely uses the processor (2.53 dual core, shouldn't matter) when playing DoomRL. Part of the Dwarf Fortress problem is the lack of "down time". Since the game progresses instead of waiting for a keypress, there are squares that change even when the game is paused and DF is only single threaded the interface has to work a bit differently, which leaves a lot of room for inefficiency. Assuming the display screen->accept keypress->display screen->etc. paradigm is kept, it's easy to write efficient code with only one or two threads (I like threads though). Even if that paradigm isn't followed, good code can still be written, just won't be as easy.
Either way, it'd be cool to play with an ASCII tileset, mainly so the window was a touch bigger on my screen (hopefully).
