First, I haven't actually begun rework on the port replicator, so that's no time wasted whatsoever. Procrastination pays! ;D
Second, I reread your post while I'm actually awake and see that you've already covered the point I made last night. :I
Third, I downloaded the Service Manuals for the 575 and 580, interesting
little board that 575! :approve:
I see why you might want to give the 575 MoBo a try. Have you got one already? Is it safe to assume that the CC Crowd has already figured out how to adapt the Analog Board connector signals(?) to provide VGA output? That's the only fly I see in this ointment for your VGA-Classic. ISTR some of the CC hacks including a display upgrade to 640x480, so that might not be much of a problem in that case.
The power adaptation doesn't seem like much of a problem . . . 8-o . . . hrmmm . . . }

. . .
. . . now that I think about it, eschewing use of the metal chassis and replacing it with a custom plexi chassis, as we discussed, opens up the possibility of stealth mounting a 630/6500 MoBo! By angling it up from the bottom front (MedusaMess) to approximately the level of the power switch (backplane) with the board inverted, there should still be room for the FDD! The PSU would need to be mounted sideways, just above the port replicator with the KVM switch in front.
Using the 580/630 board sidesteps A/B signal conversion, retaning 800x600 VGA output, something lost(?) in going the 575 route!