Having just started our AONE4 last month ex-MRU, I can confirm that LHR-MRU-LHR on the same aircraft is perfectly doable, although it's a shame when you catch a glimpse of all that tantalizing coastline and beaches out of the aircraft window.

We arrived just before 7am and wandered through to the immigration hall. Just before the immigration desks, on the right hand side, is the transfer desk. It was unmanned when we arrived - I spoke to one of the handling agents and within a few minutes, her supervisor arrived with a thick wad of tickets, itineraries, invitations to the lounge and boarding passes for the return sector to LHR and even our onward journey from there. She even escorted us through security back into the departures area. (Now that's what I call service!) ^

All this took less than 15 minutes, so we had a good hour and a half before boarding for the flight back to LHR, so we were free to wander through the duty-free shops offering a choice selection of stuffed dodos.

So, perfectly doable as a double-header but definitely worth stopping off for a few days, IMHO!