Haven't tested it to see if it's broken, but this is already in game. If this isn't happening you want to file a bug report.
From the Issue 17 release:
Chat Hidden: If a player is in a window that cannot display the Chat Window (such as the respec screen) and does not have an /afk message set, the game automatically responds to any /tell with "Chat Hidden".
"Mastermind Pets operate...differently, and aren't as easily fixed. Especially the Bruiser. I want to take him out behind the woodshed and pull an "old yeller" on him at times." - Castle
Thanks, Oldeb. Doesn't seem to have worked for me. I'll look into it.
It drives me nuts when I'm at the tailor or respec'ing only to find out folks were PMing me the whole time. Sometimes they think they're being ignored.
You can't use the /afk command because it disappears as soon as you click the tailor or what have you.
Could it be set up so that, once you start at the tailor's or start a respec, the game automatically responds that you're afk? Would save some hassles, to be sure!
P.S.: Radiation melee. Just sayin'.