I've been thinking about doing a 2JZ or V8 swap in my car, and I've been thinking hard on how to solve this problem. Since, I'm far away from ready, I'd like to share a device I found that could potentially be the silver bullet.
An industrial controls company named Axiomatic makes a
J1939 protocol simulator, that can convert a digital signal (like RPM) into CAN. The device has user programmable software to custom configure the CAN message, so it might potentially be compatible with the toyota / subaru protocol.
http://www.axiomatic.com/TDAX030530.pdf
Can't guarantee that this will work, but it seems like there's devices already made to solve these types of problems.
When I contacted the company they seemed to confirm my thoughts, but when I told them I only wanted to buy 1 unit as a test, they didn't seem too thrilled. They're probably an OEM supplier with expectations of selling 1000's.
Hope this helps someone, or points them in the right direction.