The thing is though Id prefer to do things through the amiga os side, this is why openpci seems the best option, Id like any work done to work on both amithlon and any other amiga system using openpci. Given amithlons direct hardware access when not using linux drivers Id imagine it'd be a "cleaner" way of doing it as well. Pretty much all the framework is in place. There's already stormmesa and warp3d (waprd3d does indeed use a subset of gl).
I don't think OpenPCI would be used for more than to find the gfx card base address. After the base address is found, the gfx card register and memory accesses should theoretically go to the correct place. It would be possible to read a gfxaddr ENV: variable also as Warp3D did with the PPC version. This is not currently working with the 68k W3D libraries by the way. I checked. I didn't see with a quick look how the gfxaddr of boards is located in W3D. The OpenPCI.library, pci.library, prometheus.library etc. are not opened. The board base address is probably found through P96 or CGFX. A 2D P96 or CGFX driver that allocates it's memory directly on the card and possibly also has other restrictions is likely necessary. If you had a 2D P96 driver specifically for that card running directly on the card and good 68k emulation (some cards require 32 bit accesses for example), then I would say, give W3D a try. There's a lot of things that can go wrong though. Why not switch to AROS where Wazp3D can give hardware acceleration?
To my knowledge its as "simple" as writing a driver for warp3d (wazp3d is probably the better bet as its easier to change if need be).
Writing a gfx card driver is anything but simple. A fully functional and mostly bug free gfx driver will take many man hours to complete. The Warp3D "driver" functions are not that difficult to reverse engineer for a driver and the data structures are already defined in Warp3D.h. The Wazp3D route would probably be better for most programmers and it has less bugs than the actual Warp3D drivers.
