That's increasingly common, e.g. going into the likes of YMML FIR from the north you'll be logged on and established on to CPDLC well before the boundary (probably already talking to them on HF as well). However despite already being logged on CPDLC and checked in via voice you always get the CPDLC "request posn report msg" after you have entered the FIR. I believe that is due to ADS-C protocol, rather than being a requirement for CPDLC to start working.