Your idea would probably work but I would have concerns about the amp capacity of the outlet, it's fuse and wiring. The outlet also needs to be "always on" and not require that the ignition key be on or in the ACC position. Personally, my advice would be not to do it that way. A direct connection between your battery and the maintainer would be safer and more efficient way of using the maintainer.
Nonetheless, I suggest that another and, IMHO a better, approach would be to install an LSL's Trik-L-Start or Amp-L-Start. Both are designed to do exactly what you want to achieve, the Amp-L-Start being a higher capacity version of the Trik-L-Start:
LSL Products / LSLProducts.com - Problem-Solving Gear & Gadgets
Once installed, it's always there, ready to go. It will also work if you have, or ever install solar panels. I have an Amp-L-Start.
