The next hurdle took the better part of half a year. Mostly as I shelved the project after unsuccessfully getting the PPC to execute code. But a posting on the English Amiga Board (http://eab.abime.net/showthread.php?t=76633) resulted in help from someone who had some success starting the card up on a modified Prometheus card The Prometheus (and most Mediators) are missing a 3.3V line. Well, lucky for us the Mediator 3000Di has one!
It only took the flipping of 2 bits to get the PPC to execute the code set up at $fff00100 which is the default vector of a PowerPC chip after reset. After re-reading the MPC107/TSi107 manual I actually found the paragraph stating the way to negate a reset of the CPU. Strangely enough, this was not mentioned in the example on how to boot the CPU but in a different part of the manual.
This person was also kind enough to share the code he lifted from the Mac driver for the Sonnet to initialize its RAM. So after a bit of tinkering we finally had code running on the PPC!
I don't have any meaningful pictures of it, as the only thing it does it to move bytes around in memory, but what was accomplished is that the RAM of the Sonnet was made to be recognized by the Amiga. This is done in the same manner as how the excess Video RAM is recognized and used by the Mediator software.
Next stop is to try to run software on the Sonnet which gives some meaningful output to a console or screen. It also would be great if this could be achieved using existing WarpOS software.