In our local set up, the echo360 tool uses a unique identifier ("identikey") that is a cross-campus unique id. That key was already used for the campus-wide D2L setup, and we're trying to set up echo360 with our departmental Moodle setup.
I've gone ahead and modified line 248 of mod/lti/locallib.php to emit "username" (which we set to be the identikey identifier) rather than the internal moodle ID (which is the row number from mdl_users, best as I can tell).
I'm uncomfortable keeping this change in our code based, but we can't control the echo360 install because they don't want to break integration with D2L.
What remains to resolve https://tracker.moodle.org/browse/MDL-44902 to allow such integration?