TSF,
One of the mechanisms we use when we submit design concepts or samples at no charge is a combined Receipt and Memorandum of Understanding. This document is in effect, a contract which very clearly spells out our claim to reserve all of our rights and explicitly defines the limitations of use of the concept and/or samples by the client in the absence of a subsequent contract. By signing this document, the agent of the client acknowledges receipt of the concept/proposal/samples, and attests that he/she has read and understood our terms of use and agrees to abide by those conditions.
This document has been quite effective in preventing some of the things you describe, such as developing our concept in-house or showing our proposal to one of our competitors in an attempt to get them to underbid us.
As for your idea about the legal advice pro bono thread, I think it's a great idea if the admins are willing to support it, and providing that some of the attorneys out there are willing to volunteer. Absent that, it might help to have in general a sticky thread on the skinners board with links to information on copyright and other IP rights, and maybe links to legal forums where they could seek advice.