Most likely not possible in IV:MP. But perhaps a manual /afk command could be added, so that players could toggle their AFK status. Not sure how useful would that be, though.
If you set AFK, the game could automatically answer to whoever PMs you with a generic AFK message, the TAB list could have an extra row to show the AFK status, and I don't know what else.
But i'm not sure how useful this could be, and at the same time you don't want it to be misused...