Yes, because text chats are the same thing... In the middle of the game I love hitting my PS button and navigating to my friends, scanning for the one who sent the message hitting the read button then hitting reply and typing something out with the d pad... I'm sure the rest of my opponents in MAG would understand...
Also, how do you do crossgame invites on PS3? I haven't been able to figure that out, just got one a couple months ago.
You realise if you're in the chat room, you can press the PS button which goes straight back to the game, then if you press the PS button in game it goes straight back into the chat room? Very helpful, especially when friends don't have mics.
And to invite someone, just go to your friends list, press Triangle, and click invite to game. Or the game you are playing might have an invite system, like in Bad Company 2 you go to Squads>Invite to Squad.