It is where you book the flight that matters. It has nothing to do with your flight origin. Maybe you are thinking about married segments availability which is a totally separate issue.

If you book LHR-MRU-LHR with BA MRU, then the availability would be the same which is probably none. If you book LHR-MRU-LHR with BA LHR, then it should be available. The problem is how to explain to BA LHR that you want to queue it to MRU when you are originating from LHR?? The reason why it is available when booked through BA LHR is yield management correctly presumes better yield through BA LHR office. Also, the whole itinerary needs to be re-queued to Dublin for pricing as you are changing the origin.

Bottom line: yes, it is possible. But you need to be convincing and/or to get a dumb BA agent at LHR office.