This doc has all of the Server REST Interface information, the right has links to all of the areas you can control through REST. Unfortunately I don't see anything related to license assignments. The only semi-related area is the Server Management section but this only allows operations on the server license, not control over license assignments. That definitely sounds like it would be useful though so I would open up an RFE/Idea.
James is correct. There is currently no REST API for license assignments. The product team is working on some major improvemnets when it comes to licensing for the upcoming release. Would you mind sharing your use cases so that our product team can comment on whether that mgiht be supported with the new licensing features we are building?