My experience has been that it can be a bit of a pain to get them to release the seats ... but it is totally possible. Check the seatmap for flight and you'll see that there are loads of Y+ seats free then call up United (in Dublin) and they will probably initially refuse to assign you a seat because "none are available" .. then say that you've checked and loads are available when you look it up on ual.com ( just pretend to make an unrestricted economy booking ) .. usually at this stage after some checking with the supervisor you magically get assigned an E+ seat ... I guess this is only a problem for non-UA *G/*S card holders.