I am curious if one of the existing cloud platform/PaaS providers allow transferring ownership of running instances between accounts. One use case would be a service provider who is running an instance on behalf of a client could transfer the instance to the client's own account once the client wants to take over the management themselves.
I dont think currently any provider provide this functionality where instance can be directly transfer to another account but in AWS you can not actually transfer the instance of ownership but easily can migrate instance to another account by creating AMI.
Linode.com provides directions on doing it, basically using rescue boot images and
dd over SSH. Linode also allows you to create an account for others to do things on your behalf via the Linode web interface, so for your potential use case it could be that the 3rd party creates a user on their linode account for you/your group to use. When your services are complete, they can disable that account, and the VM hasn't left their control/account at all.
FWIW I'm a very happy very long term customer of linode, no other association. I just think their service rocks, as does their customer service.
I think a Jelastic PaaS is a solution for you. Its environment transferring feature does exactly what you need - re-assigns applications/projects from one account to another. Also, consider the collaboration feature, which allows sharing your environments with other users for joint development. For example, the use case can be the following - you create a base application, team up with another user for collaborative customization/tuning, then transfer environment to that user entirely.
Here is a short video with the platform functionality and UI overview, if you are interested.