Hi Dave,
Is it the standard Girling with both inlet & outlet on the same side of the cylinder ? If so, I think you might struggle with it upside down because air will float to the top of the cylinder. I suppose it depends what sort of angle you can get it at, but from memory I wouldn't be confident because you don't need much air to make the brakes feel awful.
If it's the lockhead one where the outlet is at the end of the cylinder, then as long as it's pointing upwards I think you'd be ok.
Alternative idea - you could always fit the servo in the front by the spare wheel and run a vac. tube from the engine through the central tunnel. With an extra non-return valve in the line by the engine there shouldn't be too much lag, maybe the first time you brake but after that it should retain vac in the line for a while.
Brian