I would not recommend making a release strategy depend on the users for several reasons:
- You replicate the work, since you have to change authorizations and the release strategy determination everytime a new user enters the flow;
- Temporary work assignment of authorizations isn't enough. Usually when someone goes on vacations you give authorizations to someone else for a specific time frame. Doesn't work if you use the users to determine the release strategy.
- You add a user to PO authorizations but forget to had it to the release strategy and you start getting non-releasable POs in the system (serious risk).
It's much simpler and maintainable to use different purchasing groups and/or document types.