Seconded commands from mainline have always been on the table, which would mean similar prospects to the GSS co-pilots, ie. less opportunities for an upgrade.
Those who have applied for the RHS of BA European may yet end up on the mainline list with access to the rest of BA if they want it, depending how negotiations go, I suppose. That in itself is probably a good incentive to be there.
However, as you suggest, I can't imagine a great number of applications for the RHS of knackered old 757s with little prospect of a command. Still, horses for courses......
Edited to add: Even if the BA European pilots end up on the mainline seniority list, BA European will remain a junior fleet which means an upgrade is likely to be possible in a reasonably short time (similar to, if not quicker than, the 737 at LGW) and significantly less than the fifteen or more to a longhaul command at LHR. If the BA European pilots aren't on the seniority list, the seconded commands from mainline will prevent many of those swaps. Ironically, a command on a BA European 757 is likely to be quicker if the pilots are on the mainline list!!