We sure could implement key-codes for transfer and like, but:
-- to make a transfer with keycodes PBX should be in the middle of all RTP channles to filter out DTMF codes. It means that all calls should go thru Media Server, that in turn doubles traffic. Are you ready to sacrifice your bandwidth to have an ability, which can be easily done with one-button-Transfer-capable phones?? I don't think so.
That is why we have only call-pickup keycodes, it doesn't need DTMF filtering. And we're ready to extend keycodes for functions that doesn't involved DTMF, i.e. they work when you pick up phone and dial some code NOT DURING an active call.