I'm curious as to whether anyone has done this in order to get a wider choice of tyre, particularly in terms of larger tyres? And if it's compatible with the frame geometry and fold?

You know there's really a wider range of good tyres in the larger size, right? Not wider, I grant you. There's no issue with the fold, but I guess it could lower the BB height a little, though I have read that with the fatter tyres on, there's no appreciable difference in diameter. If that's really the case, I might go for 349s on my Mini Moulton, where skinnier tyres would suit the intended purpose.

I don't know if it's possible. But if your reason for doing so, is the wider choice of tyres, I think you might reconsider.
As I have now Kojaks on a Brompton, but for the winterdays I might change to Marathon, or maybe Marathon Plus. Although the original Brompton tyres, or the Panaracer, or Stelvio, still for sale at sjscycles or the el cheapo's from raleigh etc.
So you make me curious, what tyres do you want on your Brompton?
BTW did you try the Bromptonforum on Yahoo?

You would be likely to have chainstay clearance issues with 305 tyres.

Yea cannot fit any wider tire on, any 1.25" 305 tires, ? ..
Oh and the pedal strike might be an issue .. if you lower things too much..