Is there any way to use the default system font with SDL_TFF, without writing platform-specific code? Currently I just use one of the GNU FreeFont's, but I would like to be able to use the default system font.
Well, what is "default system font"? I think all modern OSes allow users to customise fonts, so you need to query it somehow, if you want to use exactly that. If you don't want to write platform-specific code and want to avoid non-portable assumptions, then providing your own fonts is a correct thing to do.