Yes the games can usually revert to an older version to communicate, but the issue is how the Pokemon is dealt with. If it is in the party, you can't simply pretend that it doesn't exist to communicate.
And yes, the latest patch would be required for online play. That's how pretty much everything works.
Another minor issue is that the Easy Chat System has the names of all Pokemon in it. If someone with the new Pokemon sends out that message in the Union Room, puts it in Mail, uses it as a Battle Facility quote, etc. then how would the other player receive it? Sure, they could get rid of the system altogether and allow players to simply type, thus completely destroying inter-language communication.
It may be possible to set up, but why would Game Freak go to all that effort when the current system works perfectly well? Sure, we find out about them early, but a large part of their audience doesn't.