We are looking into this problem as well, a lead from the Tom Tom to the Autocom sounds good if you can get one, failing that I was just going to stick the TT under the clear plastic on the tank bag hoping that it would then be waterproof enough. Our thinking was that you only really need the TT when you actualy get to you destination to find hotel/motel etc.
Problem is if it's raining your visual sight of the TT is going to be bad anyway as the water will still be on the top of the tank bag, and at my great age I am more of a fair weather rider if possible!
A Tom Tom Rider looks OK but do you want to go to the expense of having 2 Tom Toms?
The other thought we had was as we have an Autocom, Trudi could hold the Tom Tom on her lap as we approach the town we want to stop at and can tell me which way to go through the Autocom, although again of course the rain problem is tricky.
We have a mate further down Spain near Gibraltar who we are visiting on Sunday, I will have a chat with him, and take a few photos 'cos I think he has made a small mount for his... will ask about the rain problem too
Zippo