For some findings to report:
> drained the oil. There was a LOT in the sump but I'm not sure how much would be normal. Oil stinks of gasoline. It seemed to be thinned, to be sure, but not to the extent of "watery." No metal chunks or anything came out. I still couldn't move the crank after draining the oil.
> looked at valve timing. The timing notches don't line up with the index lines on the post of course. I made a little gauge to see how far offset they were from the index mark. Both sides were offset by the same amount, so it didn't look like their relationship with each other got changed.
> borescope:
* looked in all the cylinders and saw nothing obvious amiss. Stupidly I left the 90deg angle mirror at home so couldn't see everything or look at valves. I'll do that tomorrow.
* looked down the chain path as far as I could. Nothing obvious - couldn't see broken teeth, metal chunks, anything like that.
* ring gear: poked as far as I could (which was pretty far) and couldn't see any damage.
So, it does look like there is some kind of mechanical blockage. Or maybe hydrolock lead to something else? The mystery continues.
Next step, I guess, is to start >>really<< taking stuff apart. Seems like 3 choices - from the rear (transmission), the bottom (oil sump) or top (heads). Any further theories or suggestions are certainly welcome. Seems like pulling the sump is probably less involved than the heads.
Oddly, a sense of calm has overtaken me. It's so baffling that I'm now philosophical about it. Cheers,