The idea is for slight tweaks that they consider to be beneficial can be packed up in a diff and given to one of their many developers with cvs commit access, but if your tweaks aren't considered beneficial.. yeah, I guess you are a little stuck.

As for custom clients, there was a discussion of this earlier on the forums, and the project director laid out a plan for beneficial modifications to the client to be reviewed and approved for use on the public server.

Hope that answers some questions,
josePhoenix