Aha, interesting! I'll say I don't know of a way to get what you want through the Canvas LTI setup right now, though I am only a Canvas admin and not a full-fledged developer. With most LTI launches, the parameters sent are about the current user launching the tool. In your case, you probably need the current user info, but also will need extra info about the target user as well. I can definitely see use cases for something like that, but I also think it would be a bigger undertaking to get Instructure to implement. I know University of Michigan has been asking for some additional LTI placement options for quite a few years, and there hasn't been much action from Instructure yet. I just bring that up so you know and might want to investigate alternatives if you're looking to develop a tool somewhat quickly.
-Chris