Is it possible to buy a frameset from Trek that isn't listed separately?
No, Trek will not sell you just the frame if they don't offer it in their product lineup. And a bike shop wouldn't either, given that they have to buy the whole bike anyway. It's just a massive headache to allow things like that for manufacturers. And many manufacturers generally aren't all too keen on letting people sell their bikes online (accessories are fine to some extent). 

Moreover, why would you put Di2 on a crossrip? It's frame isn't designed for it (e.g. mounting the battery), and its quite low end compared to what people would run Di2 on (a Di2 groupset costs more than most of the crossrip models out there, as complete bikes!)