It's unfortunate if that's really an AA limitation. I did credit an AS marketed and operated ticket once with one segment credited to AT (Royal Air Maroc) and the rest credited to AA. (And I know it's not oneWorld, but I've done it a number of times on UA where a single segment on another ticket, such as TP or LH was not eligible for AC miles, so I credited single segments to UA.)

I'll be following along to see the resolution.