Yeah, that's what I thought, and exactly what I'd be doing.

So, I'm going to book the 2-4-1 LHR to IAD tickets online at T355.

Then I wall call BA at T355 when the MIA to LHR flights open for booking and ask the agent to add that segment to my existing booking to create a return trip.

Unless anyone sees a problem with that approach that I'm missing?!