Based on the pic of the dev board, it certainly looks easy to add PPC to an existing accelerator card.
I wonder why more PPC upgrades weren't made this way?
I guess, it had to be the fact that back then, the Amiga still had possibilities of a future (1996-97), so this device was designed not as proper accelerator per se, but a hack for developers to tinker with PPC, and get their software going, until better production models appeared.
Today, drop in the market whatever Phase 5 did, and you will still get people interested and lots of $$$.
And dare make a PPC accelerator for Amiga with any speed and you will make plenty of money (That is if you can cope with the costs of R&D).