Is it possible to buy a frameset from Trek that isn't listed separately? I am thinking of buying a Trek CrossRip or 720 touring bike, but I want it with Ultegra Di2 and the bikes are only listed on the Trek website with lower-end components.  So I'd like to buy a frameset and build it up the way I want...but how?  I don't live in the US and the official Trek LBS I've been to is outrageously expensive for everything, so ideally I'd want to buy online.  The Trek 720 is a brand new model so finding a used one in the right size seems very unlikely.

Some brands list framesets on their websites separately, including Trek, but the particular frames I'd want aren't listed this way.
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!)