I thought about resource engagements and it does provide a way to have the RM assign resources to projects. I didn’t say anything because from your question I didn’t think it was an answer for you. Any tool out there will be based on the project and not an individual assignment. If that is an option then resource engagements might help. Good luck, and I don’t have a better idea for you. Again, the tool is “primarily” about the PM owning the project.