Great DIY.
I used the same Nexus 7 (1st gen) + Galaxy SIII combo when I bought my car and drove it across the province, the phone as a hotspot, and the Nexus 7 as my GPS Map. Unfortunately, I didn't have wireless charging, and had both my devices plugged into a 3-way USB to Cigarette plug deal, and one or the other device would drain of all life.
I haven't thought about wireless charging or a DIY like yours, but I'm in the market for a 2x USB plug that' have anything higher than 1.1A output on both sockets.
ThinkGeek.com has a 1xUSB plug that's 2.1A, but that'd still mean I'd lose one device or the other regardless.
What's the wireless charging setup like in cost?