As I have various AA flights operated by Alaskan in F coming up I thought I'd have a go at getting something in writing.
A further call to EC customer services and a long wait followed by "We are still in disagreement about tier points for an AA flight operated by Alaskan. Can I call you back sir?" She called back and confirmed AA prefix flights will always get AA benefits, tier points and all. I explained how hard it is to get credit & she sent me an email confirming. (Itinerary deleted) as follows-
Thank you for using British Airways on-line. (did'nt)

Your Notes
--------------------------------------------------------

have spoken to membership enquiries and they have confirmed flights with aa
prefix will receive american airlines benefits

Your Itinerary Details
--------------------------------------------------------
...not exactly what I'd hoped for but I think it proves entitlement to tier points as well as miles on codeshares.