How fast is the ROM going to be on the P2? Same or slower than the Hub RAM?
Might it be simpler, and more usable, to map the ROM into the upper part of the HUB (above the 512KB and below the 1MB)?
You would just need 'n' blocks like the RAM.
If the same speed, then all code could access the ROM. A high volume customer might wish to get a new ROM mask done for some P2's, or you might like to add something for a later ROM revision.
If the ROM was slower than the HUB RAM, then at startup it's already using a slower clock, so booting can still be done using the ROM in situ. For anyone wanting to use the ROM, it would be a simple matter to start the oscillator at say half speed, copy what they want from the ROM to Hub RAM, and then increase the crystal speed to 100%.
BTW since you are now obviously talking to better people at OnSemi, it may be prudent just to ask about FLASH again, just in case it is simple (which it should be).