Are there established models for releasing an initially proprietary piece of software under FLOSS conditions after a defined period or a certain point of time? The main problem here is that all parties involved must be able to trust that the Open Source licensing will actually take place at the defined time and no party can further defer or cancel this process.

Clearly such a model has its problems, for example it's problematic to deal with contributions from "outside", legally and technically. Ghostscript is a prominent example where a deferred model has been used and abandoned. However, if certain parties involved will insist on keeping the software proprietary, at least for a certain period of time, then the only options are a deferred Open Source licensing model or no Open Source licensing at all.

I think I read about services that serve as trusted parties who take care of Open Sourcing the software. However, I was not successful in spotting any of those.