I have looked into the same thing, but using AS to BA for a cw ticket to thr for 100K off peak. No mileage accrual though.

Then again, it depends what you are trying to do. If you are attempting mileage accrual to maintain your PLT status, I would fly lax-lhr on AA and switch to either IR or BA in LHR. I think AA can sell you a lax-lhr-thr ticket. That way, you'll get full mileage accrual. I'm not sure 80K will get you more than a rt y to j upgrade for your lax-lhr flight. I don't fly out of LAX, but I'm not sure what the loads are like for mileage upgrades.

Good luck and please report your findings. I'm curious to hear what you ended up doing.

lala