LTI Custom Fields - Variable Expansion (Substrings)

[rtoon]

Does anybody have any ideas on sending over custom fields to an lti integration that contain substrings of an existing variable? I tried this to hopefully get 2 characters starting at the 10th location, but it just sent the entire field as a literal.

student_campus=${Canvas.user.sisIntegrationId:10:2}

I looked at https://github.com/instructure/canvas-lms/blob/master/lib/lti/variable_expander.rb#L26 but that's outside my knowledge zone right now.