It looks like you're using exactly the same 40 pin->CF sled that I ordered for my crack at the CF board, that's reassuring. (From the chip count I'm going to guess we're using either the same or very similar circuits; my board of course lacks the ROM because it's on the RAM card.)
Well, I've had good results with this one. It's also a good size to measure against.
I do wonder just how much abuse the built-in power supply can take.
It could probably handle one or two extra cards without much issue, but if you tried to piggyback a hard drive onto it in addition to the two built-in floppies? You'd probably run into some issues. According to the datasheets, the power supply is rated for 3A on +5v, 1.25A on +12v, and 0.1A on -12v. So just need to put the multimeter to that and figure out what it's drawing.
If you did want to add an additional 5v from an external regulator I'm a little curious how exactly to make that electrically kosher
Remember when peripherals came with a grounding tag on the cable? Chassis ground is the signal ground as well (aka, common ground), and that's what the voltages are biased against. I'd probably have a switch, or perhaps a barrel jack for the power brick that has a switch built in, so you can disconnect the bus power when using the brick, but so long as the ground is common between the two supplies, I don't see any issue. Without the switch, the voltages would need to remain exactly the same, else one would be feeding into the other, so being able to separate them is the best idea.
The Tandy doesn't give out -5v, and not much uses -12v, so I wouldn't worry about those, but a +5v regulator from a 12v power brick would allow an external hard drive to be run in that case as well.
I've been thinking of getting myself a 3-D printer at some point, if nothing else, for replacing plastic parts that are hard to find (like for my dishwasher). Now I've got a good project idea for it. Probably make it large enough to house 2 or 3 8-bit ISA cards, or a 5-1/4" floppy drive, with ribbon connector knockouts so you just knock out the one for the ribbon you need.
First though, got to finish up my current projects.