Dont forget Toyota have the Lexus range of 4wd SUVs to gain experience and technology from. I believe they were electric only on the rear axle ?
The Lexus SUVs which are electric-only on the rear are in my category of AWD "crossover" style vehicles (including the Lexus RX and NX); the conventional-layout (longitudinal engine) Lexus hybrids have not been SUVs. But it's a good point that the Lexus GS and LS hybrids used a longitudinal hybrid transmission, which might work as the core of a Subaru-format transaxle... although at huge and unjustifiable cost. That still leaves the AWD system: separate motor for the rear (like the Toyota/Lexus crossovers), or Subaru-style?
I am really curious about the design, because in the previous Subaru/Toyota effort (the Subaru BRZ / Scion FR-S / Toyota 86) the model-specific design and development effort was absolutely minimal, using nearly all Impreza parts, but with a Toyota/Lexus transmission and final drive.
The RAV4 EV was a Tesla-motored compliance effort, unrelated to the hybrid designs.
But Subaru will want to capitalise on their famous 4wd tech and probably keep the electric drive all integrated with the ICE as in the Prius....and just a mechanical rear axle.
Just a SWAG.
Subaru doesn't have any 4WD tech that everyone else doesn't have as well, but yes, they could use a mechanical clutch drive to the rear (which is what automatic transmission Subarus use), or a centre differential (which is what manual transmission Subarus use). The AWD Lexus hybrid used that longitudinal hybrid transmission with a "transfer case" differential unit on the back, driving a front diff tucked beside the engine in a way what won't work for the boxer engine. I doubt the Crosstrek will have any performance pretensions, so I think they do whatever is cheapest, and allows the packaging of a battery.