Using RESTful API to add a user to a role in an app

I'm writing a Python script and would like to add a user to a particular role in one of my apps. She is already provisioned in my company's realm, and I have her user ID number.

I see that API_AddUserToRole exists and is well-documented, but I want to use RESTful APIs rather than XML. I see that there is a RESTful API for adding a user to a group, so I have to imagine there is something similar to add a user to a role in an app, but I can't find it. Any suggestions? In particular, I'd need the correct URL to POST to.