Even the Early Startup screen doesn't display when hooked into the pass-through (monitor flickers for a second and says something about "cannot display this analog mode"), so I don't think it's software-related.
- If I pull the cable going between the Indivision output to the PII input, then (and only then) will the PII display the Workbench.
...I hooked up the 23-to-15 pin silver C= adapter and plugged that into the input on the Picasso II, and switched to Multiscan mode. Display came right up. So that solves it, the PII input and switcher work fine, but don't play nice with the output of the Indivision ECS...

I also haven't had the best luck replacing CRTs with LCDs. The LCDs seem to clock differently and are more particular about the input. I have often seen that message "cannot display this analog mode". With much more modern graphic cards, those Blue-shelled monitor cables have saved the day with Debian.

They are probably a problem for Picasso era cards, though.
I know you don't have any old style CRTs. Right now, they are plentiful in Thrift stores for next to nothing. Next year, they might be hard to find (There is a panic on to recycle them). I'd be interested how your experiments come along with a CRT, even if you don't intend to ultimately keep it in this system.
You have tried other (different spec'd) LCDs?
On the bright side, lots of folks have jumped thru hoops for the novelty of having dual monitors with their computer.
