You got me going with that one, how about going whole hog and just using a harvested PBX ASIC underneath a G3/RAM CPU card? My guess is that the whole thing would run just fine off 2300c or 1400 ROMs? [}

]
Ah, you read my mind! This was "Plan B" in the event that our current "Plan A" failed. Just like you said, expand the CPU daughterboard into the modem space, to fit the extra RAM, bringing it up to 32 MB on the CPU daughterboard, 64 MB total for the BlackBird. It should easily be possible with the space available.
The TI PBX on the NuPowr 167 board is literally identical to the one found on the motherboard of the 5300 (and I'm fairly sure, the 1400 as well), down to the Apple assigned part number. So, apparently, the "Pratt" from the BlackBird is fully forward compatible with the the "PBX" from the 5300 and the 1400 (I'm even starting to wonder if PBX actually stands for something like Pratt, revision B, eXtension.)
The ROM is the tricky part. I'd really like to dump the ROM that the NuPowr card is using, and compare that to the original High/Low ROM on the 68k CPU daughterboards for the BlackBird. I have a funny feeling its essentially the same between the two. The NuPowr card also has a Xilinx chip that is not too far from the CPU, I've seen it labeled "OF 34" which I am betting is "Open Firmware 3.4" since that would fit the time frame, and the use of Open Firmware with PowerPC systems. If so, one would need to use a High/Low pair of ROM's, as well as read out the contents of that Xilinx chip and either modify it as necessary to program a similar chip or just transplant the actual Xilinx chip onto the new card design.
The CPU, RAM, ROM, PBX, & the mysterious Xilinx chip are essentially all the major components to the CPU daughterboard. The rest are simple support chips, voltage regulators, little caps and resistors, etc...
I'd really like to see the support chips being used on the G3 upgrades that were made for the 1400. All the images I can find online for G3 Powerbook 1400 upgrades show the support chips covered by stickers, so I can't read what they say.