I mean, surely the GPL will be honoured and the source will be made available to those you have distributed binaries to on request...right?

So, there's no harm in discussing your changes publicly.
Depending on the changes you would like made, I would be happy to consider this project.
But I do have some conditions of my own:
- GPL requirements are met.
- The "credits.rock" plugin stays in place in sources and distributed binaries.
- Monies negotiated for payment of the initial modifications (and further updates to the sources) are to be paid directly to the Rockbox Development/Beer fund.
- ...possibly other things, depending on what it is you're wanting to change exactly.
[St.]