I think StormMesa had a software renderer... not 100% sure, but I dunno if OS3.x SDL can use it, but really, i reckon a software GL rasteriser on 68k would be a slideshow.
The thing is, on modern systems, 3D APIs can accelerate 2D as well as 3D; just look at the accelerated interfaces on Vista and MacOS X. Giddy got rotation, scaling and alpha blending for free from GL.