Tell them it's OSS and they can fork the code-base if they want more fine grained control. I know it's not going to resolve any issues but at least you are forcing them to say why that's not possible now and what long term plans they might have in store.
Well, the question is about the authorship. If the company holds the copyright, and always held it based on whatever is in the employment agreement, this will not work, since they can just relicence the project, and the original licencing of the work under OSS license would also be in question.
Ownership might also be the wrong terminology, but I wanted to avoid the word "authorship" because you can never relinquish that right. At least to our national laws... Again not sure how it works in the US