Looking forward to the BPL review Roger. Good to know in advance that it will probably be an issue.
In the past I have considered ways of keeping my arms on the pad (another 2 inches of width would do it!). Shoes or extra bits of pad are one option, though you need to keep them fixed in position somehow. I've also considered using a piece of elastic loop to keep my arms close in to my body (haven't tried that one though).
My favourite way, providing the temp is cool enough, is to decrease the diameter of my silk mummy bag by stuffing excess fabric under my back. That way I can use the sides of the liner to support my arms and stop them slipping off.
Not a problem for side-sleepers of course!