Linked-server query against remote DB does NOT fail when DB is in SINGLE_USER mode
Upon re-trying and attempting to reproduce this problem, it seems I cannot.  I think Nic said it best so far: "being in single-user mode doesn't implicitly prevent connections".  At the time, the linked-server query must have been the first and only thing connecting to that DB.

Thanks also to Aaron Bertrand for suggesting the restricted_user mode for future cases.