I brought home one of the gViews I had at work, along with two other ATI video cards, an AGP Rage 128 Pro, and a PCI Rage 128 card. I was surprised when I opened my G4 and discovered I had a Rage 128 Pro in there already, I could have sworn it was an nVidia card...
The two 128 Pro cards were slightly different, one had a fan, the other didn't, and were otherwise identical except for a color difference between the DVI connectors (w/fan, black, w/o fan, white). Turns out that both cards worked with the gView adapter.
I never did try out the older PCI Rage 128 card, but I suspect it also would work fine. I was really starting to believe there was some OS X issue going on that was eluding me, but obviously that's not the case.
But some questions remain, such as why the adapter didn't work on the G4's I had at the office with nVidia cards, and why my home brew adapter wasn't working.
On that last point, I think it comes down to not having the right parts, I believe the diodes I was trying to use were too large, looking at their specs I believe the voltage drop across them was sufficient so that the signal that was being measured wasn't being passed, and therefore read as an open circuit. I'll attempt to find some smaller ones next time I get to the parts store and will try to revisit that.
More details as I get them. ;)