So, I'll lead with the end goal: An rclone mount that automatically mounts a remote on router start if there is a network connection, with sharing/user management handled by GLiNet's Disk Management router admin interface.
I can rclone mount my remote with no issue, so that piece is covered. What I'm having difficulty with is getting the router software to pick up that folder as being an external disk so that I can set up the Samba share. Where is the software reading this from? I'd much prefer to manage the share via the UI, but I imagine I'll have to modify a script somewhere to get this mount to show up in the Disk Management UI.
Edit: As a "workaround", I set up an init.d script that mounts my rclone remote to a directory on the microsd card. My rclone mount is now accessible via Samba because it's underneath ../disk1_part1 (../disk1_part1/rclone), but I don't love the nested mount situation. The start() portion of the init.d script does a curl check to an external URL and only performs the mount if there's a 200 response, so the only-mount-if-external-internet portion is covered.