The best for now is to give up, and use some Linux device (like Raspberry Pi) to set 44networking.
We've been telling him that for weeks now...
Of course, now he has a MikroTik router which is much more flexible than the average router. These things (as tiny as they are) can run a virtual machine! (the feature called Metarouter) It may be possible to run a small virtual machine with Linux on it, that either does the tunnel and RIP processing, or is running the script to re-configure the main router all the time to add/remove the tunnels.
That way it may be possible to get it working even on this low-end router, with the advantage that it has multiple ethernet ports and even WiFi.
It would actually be nice to have a solution for the appliance operator to get an IPIP tunnel, although there of course are many challenges to overcome. I should find time to play with the Metarouter running plain Linux...
Rob