Doesn't seem so. I just tried it, but gave it one try. You can definitely be in clamshell mode with an external monitor AND and airplay display. But my MBA goes to sleep and then airplay display doesn't continue.
Ok, I tried second method, still didn't work.
Internal Display (open)
External Monitor connected
AirPlay Display On
then
Close clamshell (doesn't sleep since external monitor is on)
Disconnect external monitor.
Airplay Display still active and main display!!!!
oops, a few seconds later MBA sleeps. Try to wake up with bluetooth but APD is already shut off...
I guess a workaround would be to get a cheap small external display and use it for basically nothing. Then use Airplay display for most tasks.
Maybe I'm wrong, this is just my 5 minute testing...