Hi Kronicklez,
I think implementing FBF which includes Routing-Instance with event-options is a long shot. I never tried but think it should work.
1. Configuring Routing-Instance and separate default routes.
2. Performing route import/export between the instances.
3. Configuring RPM with probe targets.
4. Using event-options to deactivate ISP-1 route when it is down and activating when its back.
I may missed few steps in between because I never tried with this combination.