Quite an intriguing idea!
This is what would be called Thinking outside of the "Bivy".;-)
The way that you describe entering your bivy is the same method that I use to "don" my top quilt.
I pull the quilt over myself, insert my feet into the footbox, sit up and put my head through the drawstring cinched / snapped together opening and then lie back down. Just a little side to side rolling is all that is required to "tuck" the quilt under my body.
Am I doing the math correctly? When I convert .75 m to inches I come up with approximately 30" in width for your pad. Is this correct?
With that kind of width staying on your pad during the night shouldn't be to difficult.