Surprising as it may seem (or not), I do agree both with takemehomegrandma and Velcro_SP: The MPC5121e is a nice clever chip, but not commercially viable. So, even if it would be a perfect match for MorphOS, it won't happen.
I'd also like to say that cache coherency is not a mandatory feature for an operating system to work. You just have to rewrite every driver that needs hardware cache coherency. Ouch.
Genesui suffered from communication problems with freescale that led to the shocking discovery that the MPC5121e wasn't meant at all to be compatible with the MPC5200B. That's enough for a company to pull the plug in development.