So, got 'er done. It's not great, but it's super solid, and it fits (barely).
Got a chunk of aluminum from MetalMart (local metal supply place that sells offcuts). Did a bit of design, and marked the holes to be drilled with a carbide v-bit using my cnc. Drilled those holes (using cutting fluid and my drill press). Drill holes too big to tap with 1/4" tap, so I had to tap them 5/16", which worked OK. The aluminum was soft, so that worked out.
Once I tried to mount the thing I discovered four screws hiding behind the z-axis assembly. Counter-sunk the inner two, and then drill relief 'notches' for the other two on the back of the new aluminum piece. Attached the aluminum piece with lock-tite all over the screws into the PVC.
Then I drilled some holes in the z-axis assembly to mount it to the mounting plate. I was a little off (wish I could have used the CNC for that, but oh well, it fits). Four bolts (with their heads ground down to fit inside the z-axis assembly), four washers, and it's in place. Nice and solid.
I had to epoxy a little tab of metal to the new aluminum mounting plate to get the x-axis home switch to engage properly. The new mounting plate is wider than the z-axis used to be. I also had to adjust my machine parameters in LinuxCNC. Lost under an inch on the X-axis (currently at 12.75").
I would have liked to have gotten a better spread on the up-down of the attaching bolts, but it seems very solid already, and I figure the stress will come when the CNC pushes down, not up, so it'll lever the z-axis assembly up towards the new plate, not away.
I would love to just replace the entire z-assembly (and z-axis assembly mounting uprights) with tapped aluminum, but that's a little outside my skill level...