Ok, so now that our server is up and semistable I've done some more messing around. When I'm logged in on Shockone, Shocktwo and Shockthree (account names are respectively Accountname1, Accountname, Accountname2), people see different things. One friend sees "Ryan Lastname - Shockthree, also playing Shocktwo" and another friend sees "Ryan Lastname - Shocktwo, also playing Shockthree, Shockone). And this changes if either they log off or I log off. But there seems to be no rhyme or reason to which player you are seen as "playing" (which is the one the RealID whisper goes to), and it seems pretty common that they can't even see all three of my characters.

Sending a RealID whisper from a character they can't see to them does add that character to the Also Playing list, but it does not set that character as the "now playing" character. So this is all very confusing and could be frustrating if I ever end up using the RealID chat. I guess I'll give it a day or two and see if there's any updates or discussion on the WoW forums that would be relevant.