It's actually ACQ that's complaining about the maintainence. They want the project to be finished, and if they need some minor changes, for me to get involved. Probably due to the cost of monthly payments (speculation).
A maintenance contract, however, would grant such a license to ACQ. And if such contract allows for subcontractors to access the source code, then formalizing SG as an unpaid contractor would be a way forward.
Do you know where I can find an example of this kind of licence?
EDIT : Basically, ACQ wants changes after delivery date, and I need source code to make the changes and deploy them to wherever. We need a licence that says "ACQ and SG will use the source code only to make changes for their system/deploy and not sell it, SWF keeps all their rights and do whatever they want with the code"
EDIT2 : I went for a slightly modified Polyform internal use licence, what do you think? https://polyformproject.org/licenses/internal-use/1.0.0