I would bet they will say no on the phone but a check-in agent might do it if there are not 1000 keystrokes required (which is likely).
Apparently it in not uncommon for other carriers (let alone alliances) to refuse to do this on two separate tickets both on the same airline, so interline or not, they may just say tough darts. If also might make a difference if the EI flight is in business class and say, refundable if someone thought it was irksome. But it won't take them long to notice Ei gets the pennies and AA gets the pounds. (Or euros, I guess).
But there is no harm asking (calling them won't amount to them arranging this for the OP) when checking in as long as both e-tkts are on paper ideally from one reservation.
I don't know why these airlines have not computed the effort - say 30 seconds - and if there is an interline agreement, adding a $20 fee at check-in, vs. refusing. Tickets on the same airline is really a particularly terrible example of putting it to the passengers, but the airlines know that at times it's cheaper to do ticket 1: A to B and ticket: 2 B to C than one ticket A to C via B and as such they are not going to encourage it.
If it's cheap enough for all this hassle, just allow for immigration, customs, luggage, check-in and security to go out and back.
People probably do this to save $50 without thinking of the effort mid-way through.