Well, it doesn't really matter. The shaders on those old cards are pretty weak, I still can remember that my old Geforce 6800 has to fold when doing a simple fullscreen invulnerability effect - with assembly shaders! The frame rate tanked to less than half of what it was without shaders. These were my first tests with shaders and caused me to hold off on them until I upgraded my computer a few years later.
I guess the same will be true for other GL2 hardware as well. It's just that Randi went to great lengths to make the D3D code work even with hardware so weak that it has completely disappeared from the market by now. I believe the oldest he targeted was Geforce 5xxx and equivalent Radeons. Of course that was 8 years ago, so today GL3 should be fine, especially since there is a fallback path (i.e true software rendering.) At some point these people have to realize that with their outdated hardware they are getting nowhere anymore. (Well, it's just too bad that most ports still see this as the baseline to develop for...
On the other hand, if you do not use any more modern features, providing GLSL 2 versions of the shaders should be sufficient, right? Today we can assume framebuffers and NPOT textures on any hardware that supports GLSL.
For the OpenGL init code, since that's already present in GZDoom and mostly separated in its own files so it should be doable with relatively low effort.