[smuzz]

Hmm ... interesting.
Since it's not one of those scenarios where you've got to make an irreversable decision, I'd suggest assembling the whole thing with the single flight battery and ESC - then tie the plane down, connect a wattmeter and run it up full bore whilst adjusting as many servos as you can simultaneously. You could (if you were keen) estimate from the motor and servo specs what the total demand might be beforehand and see if that is anything like the wattmeter readings. The advantage with doing this is that you've got it on the ground, so no disasters are going to happen and when it gets in the air the motor+prop will unload a little bit anyway so the total current draw will be a little less.
If things turn out to be marginal with the conventional setup, then Kieran's suggestion is the next thing to try. You're probably better organised than I am and so the extra hassles of another set of small batteries to manage and administer mightn't be a problem for you, but I'm sure I'd have trouble keeping track of 'em since I already get myself confused at the field when I've only got a collection of two different battery types to keep an eye on.

Sounds like an interesting project anyway Chris - looking forward to seeing it ...