I did read it - they say that they give a game all the RAM and don't get it back while that game is running. But retro actively reserving a memory block wouldn't be impossible - like I said, it's just memory allocation (malloc).
Even something small like a 500KB to 2MB allocation would do, which would just be for the process/daemon required to initiate Cross-Game Chat. The game would still get all the RAM it thinks is available and the memory that is reserved is used for the Cross-Game Chat feature.
It would only be VoIP anyway, the average VoIP conversation is about 90Kb/s (KiloBITs, not Kilobytes) which is nothing to do with RAM, that's pure bandwidth (incl. overhead).
The only thing that would need memory is the initial process/daemon to start the feature and maybe the compression/decompression of each packet in the conversation... but even that would require a barely negligible amount of memory.
Bottom line is, a simple non GUI based client which can be accessed via XMB or from within a game, which is pure VoIP, will not require massive amounts of RAM to operate.
I don't care about this feature either D3adliner but I don't like it when something can be done but the company CBA to do it coz it requires a bit of work so they just say "Yeah it just isn't possible".