Thanks muchly for the tips everyone!

To clarify, this was actually a personal award ticket due it being a family matter (so not an issue of the class booked, fare types, etc.) -- so showing them the miles accrued would be a no-go. Basically, they needed evidence that I traveled abroad for administrative purposes relating to my transport allowance, expense account, etc. as I'm going to be back in the States for the next few months...

I checked with AA and indeed, there is no process to retrieve past boarding passes. But I was able to use the Refunds website to pull up a copy of the receipt (showing LHR-LAX and LAX-SAN as 'used') and attached a car rental receipt from SAN upon my arrival, so hopefully that should do the trick...