I'm not sure about the recorded-message-then-hold thing, but Android 4.0 has a feature where when there's an incoming call, the user can hang up, answer the cal

I'm not sure about the recorded-message-then-hold thing, but Android 4.0 has a feature where when there's an incoming call, the user can hang up, answer the call, or quickly choose from a few configurable text messages to send the caller (which also hangs up the call). You could set one of those canned messages to say "Hey, I can't take your call at my present location, I will call you back in 1 minute." Not perfect, but pretty useful.