It's been a long time coming, but after a short talk with my Lead Drupal Architect, we've officially changed the idea rights in our default proposal to reference the GPL. For years before this we've struggled with language regarding mutual ownership, licensing, etc. Here's the actual language I settled on:
All rights for work created under this contract by Metal Toad Media belong exclusively to Metal Toad Media until all professional fees are paid in full. Once paid for by the Client the final work will be released to the Client under the GPL 3 License:
The GNU General Public License is intended to guarantee your freedom to share and change all versions of a program. Additionally much of the existing libraries and open-source code used by Metal Toad Media is already governed by this license.
In the spirit of the GPL, please feel free to reuse, extend and modify.