if the company uses the product under a different license, then making the change public would have to be negotiated. a consideration for the company among others is that the company thinks that the change would be a competitive edge they would like to keep themselves (although most often that is an illusion, and the extra cost of maintaining that branch is not worth the advantage. this is something the developer needs to communicate)

if the nature of the changes make maintaining a separate branch difficult then i would make it clear up front that such a change can only be made if it is incorporated in the FOSS version of the product.

alternatively, i remember from the early days of zope, one of the project owners told me that they would offer companies a 10% discount if they would allow them to make the change public.