I'm currently working on possible trips to replace my cancelled Huge Interrail Expedition - trying to find out if bus substitution services exist in reality, as opposed to a paper promise of a service is something of a challenge!
My most recent (yesterday) fantasy leg was Oloron-Saint-Marie to Canfranc.
Oh yes, it appears to both exist, and never run.
If it ever comes to making some of these journeys I can envisage some 'interesting' phone calls to local tourist offices as being the only solution.