> both the MorphOS community and the AmigaOS4 community haven't
> expressed interest in a PS3 port

I wouldn't say so. There's a great part of the OS4 community that actually wants OS4 on PS3, leading to excesses like that. And there are also MorphOS users who want their OS on PS3.
It's just that the *developers* of OS4 or MorphOS respectively stated a non-interest in a version for PS3.

> what if IBM were to change its policy of limiting access to Cell
> processors and allowed outside developers to work with the chip?

IBM does *not have* such policy. Access to the Cell *is not* limited in any way. Outside developers are not only allowed to work with Cell, IBM are actually encouraging developers to work with Cell.

> One of its few limitations [...] is the limited amount of XDR ram
> it can address.

What's that limitation with the original Cell exactly? AFAIK the PowerXCell 8i can address up to 32 GiB DDR2-SDRAM.

> IBM [...] may not be as adverse to the idea of outside development
> as was once thought

The opposite seems more likely.