To be totally honest, I don't know exactly how much information can be sent by ACARS regarding position reports.
I do know that as a minimum, an automated position report sends from your aircraft tells them the waypoint you're heading to (the active waypoint, as it's known), and the one after that. Whether CPDLC can provide more info further down your flight plan automatically, I'm not sure.
So to answer your question, yes. An ACARS position report will tell ATC you're at XYZ, at 11:12z, FL350, estimating next WPT ABC at 11:25z, and then heading WPT DEF after that.
All of this elaborate conjecture of pre-loading RTE 2 is all well and good, but wouldn't be a deal-breaker. Ultimately, if someone got control of the aircraft, the use of automation, LNAV/VNAV, hand-flying etc. is a all irrelevant.
If you lock the other pilot out of the flight deck, who cares what medium you use to navigate?
I'd have thought the big 'breakthrough' in your hypothesis isn't a clever or covert insertion of a second route. The bigger news that someone locked themselves in the flight deck in order to hijack the aircraft.