On v3.4, I was able to get everything working without issue, from your suggestions. Thanks!
It was all pretty straightforward - define a CoreXYUV, map the axes and endstops, and verify it all as a CoreXYUV, first. Then set up to the tools, where the second one maps (U, V) to (X, Y), and ensure they work as expected with T0 and T1, where each of these parks the toolhead.
Seems all good... with no pain, surprisingly, so far. I know the XY offset calibration to come will be the bigger pain.