Totally agree perfection doesn't exist and that as times change and the requirements of the customer, then so should the design and functionality of the product - whether that product has won an award or not. The actual Out-of-Game XMB should probably stay as it is, for consistencies sake, however the In-Game XMB should definitely be redesigned. But as I said previously, that is surely a debate in itself.
Accessing the Cross-Game-Chat feature is the last and least of the "problems" Sony would face retro-fitting it (as it would only need to be another option in the In-Game XMB anyway).
If someone did create a PoC (homebrew... obviously) for Cross-Game-Chat and posted it online then Sony would certainly have a problem on their hands because it would show their inability and their lack of desire to dedicate resources to developing something like this.
However, if Sony are right in claiming that lack of memory is the issue then a homebrew option may not be possible, unless a homebrew developer knows how to reserve memory before a game is loaded into it?
Like I've said before, I don't believe it is a useful or even required feature but it is a feature nonetheless which Sony should be able to implement but just wont.
Which begs the question... what else is being restricted?