The half of Sat 2 top level furthest from the terminal has been in use for years, well before the end of BAA's tenure. Go used it pre-9/11, I recall arriving first thing in the morning from Keflavik with Continental and some Lufthansas lined up along Sat 1 behind. I vividly remember parking up alongside a Transjet MD80. I think at that time it was pretty much an overspill and received a small and random selection of flights and airlines. Air Berlin used it towards the mid-late part of the decade, around seven or eight in at one time with some boarding from domestic downstairs. Ryanair have always had the odd flight from there, but in recent years they seem to have exclusive use. Bizarrely the bar which was always open in the lean years is now closed. You can see where it's boarded off to the other half, I believe it would be incredibly easy to open the whole thing up.
The lower half nearest the terminal, domestic, does indeed handle international flights. Mostly I've seen this happen off-peak, but when I flew up to GLA a few weeks back despite a couple of FR, a couple of EZY, BE and BMI filling up the ramp there was also an FR heading off to Warsaw.
If they did make Sat 2 entirely international, a potential disadvantage is that domestic pax would lose the straightforward exit out of the terminal unless a new one was designed into Sat 3 or a future Sat 4 into the new arrivals area. If they did that though, the convenience and speed for departing pax might be impacted.