Powerlet actually makes a handlebar mounted socket that has a Ram mount hole for a GPS. Bonus is that it wires directly to the battery - no splice required. Just put one on my XR650L and love it.
Doesn't solve the switched power idea, but you could easily add a switch to the Powerlet harness rather than hacking your bike harness.
Current: 93 R100GS, 04 XR650L
Prior: 73 CB350, 77 R100/7, 83 R100RT
I love every motorcycle I've owned and even some that I haven't.