As a little side note - possibly helpful if you are willing to invest more time in this project in the long term: I've had some discussions long time ago with people from wikihouse [1], and am still struggling with similar issues with my own adventures (academic research sponsored by an industry network). The model wikihouse adopted was that everyone has a seat at the table, but nobody owns the table itself (or the chairs).
It can be simplistically simmered down to: If this is a core feature that will allow your project to expand and make it relevant to a wider audience, considering labelling this as a "core feature" and insist on it being part of the open source project. They could sponsor it and/or have direct support of the developer and/or "first adopter" privileges but the code will fold back in the public branch after a given time. It would help if you had another company expressing interest (maybe just ask).
Good luck with your endeavours!