I wonder if there is a way to create standard documents like PDFs and Powerpoints that phone home quietly so you know when the document has been altered for reuse as an RFP.
There should be some technological way to determine if your copyrighted work is being used honestly.
I think the point made above was that you shouldn't care if they use your proposal/design as an RFP because you ALREADY billed them for just the proposal/design.
When I worked as a consultant doing software customisations, they were always done in (at least) two statges. Proposal and design work was a separate contract from implementation. It was made clear to the client they could go elsewhere for implementation. They rarely did.
There should be some technological way to determine if your copyrighted work is being used honestly.