My mountain to sea route was a start with the Lairig Ghru, heading up to the Abernethy Forest (either via a high route or low route) and then to the coast and back home to findhorn on coastal trail. About 100 miles. I am planning to run it on my own in the autumn - just for fun!!
Agree though, that for a route that distance, some form of support is required, although I think people would welcome a race that does not require you to provide your own support crew (well it's a personal frustration of mine when it is a requirement anyway!). So definitely checkpoints, and yes, volunteers. I think with the Centurion races they offer that if you support for them on a race, it gives you guaranteed entry to another one of their races.
Also agree, first step would be for a group to cover the distance, and perhaps smaller scale recces of sections to iron out other land issues, route issues, how many people it can cope with etc.
I always thought I would be nice to have something in the autumn, just cos the spring gets quite busy. The guy who organises the 30/60-miler over on the West coast is also working on a 100-miler ... have forgotten what it's called off top of head.