I think I have found another piece in the puzzle. On the E-M5, under the cogs E menu, there are 2 settings: Metering and AEL Metering. It turns out that AEL metering includes FEL. By default I think this is set to Auto, which means that AEL will use the same mode as Metering. But you can change it to anything you want. I've set mine with Metering set to matrix, and AEL Metering to spot. I am hoping this will allow me to use M mode to adjust for ambient lighting and meter that using matrix, but use a half press and hold to lock in the centre spot for AEL/FEL and recompose for the shot. I'll run some tests and report back.