NuBus is repaired, and the Toby card (820-0198-C) is working. I found some very curious results:
The diode rectified voltage is 3.93V.
The Toby card puts out CSYNC on pin 3, and it's a TTL level signal swinging between 0 and 4.7V, but it's not the same as the IIci and IIsi CSYNC: the period around the vblank interval looks different. First the CSYNC pulses become twice as frequent as normal, then CSYNC goes low for the equivalent of 3 horizontal line periods, then more double-speed CSYNC pulses, then finally back to the normal pattern. I think this is an NTSC/PAL thing, something called serration pulses? The other Mac video signals that I've looked at don't have this. Here's a capture:
For comparison, here is the CSYNC signal from the Mac IIci, where the CSYNC pulses are always equally spaced and continue through the vblank interval:
The Toby card also outputs sync on green. Unlike the IIci/IIsi, it does not output sync on red or sync on blue too, only green. The double-speed serration pulses near the vblank interval are also in the sync on green signal.
On the HSYNC and VSYNC pins I would expect to see nothing from the Toby card: maybe tied high or low, or left floating. But the VSYNC output is actually a copy of the green video channel! Just like the green video, it has three levels and a swings between 0 and 1.29V, which I don't think is correct since VSYNC is supposed to be TTL levels. I confirmed that GREEN and VSYNC are directly connected on the Toby PCB. Here's a capture (VSYNC is incorrectly labeled as HSYNC here):
Toby's HSYNC is not connected to anything, the pin is left floating.
On my Dell 2001FP, using sync on green (DIP switches 1 and 4 from a 10-switch VGA adapter) results in a good-looking stable image. In this configuration none of the stand-alone sync signals are connected, only the sense inputs and the RGB video signals, so it must be using sync on green.
The 2001FP using CSYNC (DIP switches 1,4,5 on) initially produced an image that looked shimmery and slightly unstable. But after re-running the monitor's auto-adjust, it looks as good as sync on green.
So... lots of interesting Toby stuff! It may be more difficult than I thought to get this working with my proposed sync-splitter adapter, because of those CSYNC serration pulses and then the lack of any pulses during the vblank interval. Self-powering will also be more difficult, since the adapter can only draw power from CSYNC on Toby, instead of all three CSYNC, HSYNC, and VSYNC on the IIci/IIsi.