This is the opportunity to write a wish list

Well, the point is the service can be minimal and equivalent to what the buyer would get just downloading it. So the contract will be "I supply you with the GPL software for you to further develop and/or deploy in your own system, and I will support it for X time including giving you the updates on a best-effort basis". (This obviously doesn't preclude having other offerings, with more meaningful SLAs).

You don't have to do much. You just have to do something minimal, and fit in with standard corporate purchasing processes.

To wit: Signing off on spending (say) €200/annum to purchase some necessary software (inc. as per above) could be something a fairly low-level manager has the authority to sign-off on within the standard processes of the company. Where "donate €200 to a Free Software developer each year" would be something not covered in those processes and require an exceptional process to be followed, that ends up on the desk of a VP or higher.

Make it easy to do within standard corporate processes, and maybe more will do it?