"...being the first 3D model used to create a humanoid Terminator."
There's precedent for that.
In "The Sarah Conner Chronicles" Summer Glau was shown as the human upon whom her cyborg character was based.
That girl was killed once they were done with her, but you figure someone like younger Arnie might could have escaped, if he were a captive and not a willing participant, and become part of, and maybe even a high-ranking part of, the human opposition to Skynet in the future (his character's present) who has been fighting it for enough years to look the way he does now.
I assume they'll come up with some way to send him back in time, although I wouldn't mind if a lot of the movie was set "then" instead of "now".
Of course your idea about him being a "Servant of Skynet" would let him play the villain (let's not spoil him by having him develop a conscience) instead of the hero (who would no doubt be someone much younger, although they should feel free to surprise me on that front), and of course the villain almost always gets better lines than the hero.