On/Off you mean something like this?
http://www.ebay.co.uk/itm/New-paintb...item5af8b37a3f

I think it would still be an issue even if I had to mount a nut in place of the knob...the flipper plate stays on top of the valve so I'd still have to cut a hole through it...

Will try to play around with the cad model to see if can somehow reposition the bottle to get the bloody valve accessible from the top or side, other than that I don't know what to do!

And you're struggling for weight as is, so adding a small motor and gear to open that valve remotely ain't an option.
Unfortunately you're right, weight does not permit that...