If I could do it again, Id probably do it a little different than I had this time.. I may still change it..
I did it by screwing the swivel mount from my ProClip directly to the dock.. I did this by using the dremel to make the back of the mount flat, instead of having the attachment for the suction cup portion of the Motorola mount... One of the issues that I came up with when I did this is that by the time you are done dremeling the dock, the plastic is a bit on the thin side. I screwed the ProClip swivel base directly the the Moto dock, putting the screws in from the inside of the dock back cover. This sworks -- but every now and then the ProClip swivel needs to be tightened, which is a hassle because you have to take the dock apart to get to the screws. I ended up making it fairly tight, and using Loctitie to help keep the tension screw from backing out.
What I would do differently is this:
1) Dremel the back of the dock until you have a flat surface in the middle to which you can mount something
2) Pick up a set of these from ProClip:
ProClip Move Clip - Move Clip Kit (1 Male and 1 Female Plate)*-*ProClip Car Mounts for Mobile Phones, GPS, and Handheld Devices
3) You can attach one side of the "Move Clip" to the base by one of two methods. On my Palm Pre, to mount a Touchstone, I used 3M Dual-Lock, which is like rigid velcro. (They usually use that for attaching toll tags to windshields) - That works very well, and will not require you to open the dock. It may be hard to find though. The 2nd method is to open the dock by getting the small rubber covers off each of the screws, and then separating the two section, and inserting the screws from the inside portion of the cover into the back of the "move clip".
4) Attach the opposite "move clip" to your existing vehicle mount.
That will allow a stationary mount, not swivel.. Im sure you can think of how to incorporate a swivel into that system if you wanted to add one (the 'move clips' are extremely handy for this type of thing.)