Since we now have "WHERIGO CACHES" which blend Wherigo and Geocaching, there is no reason why we cannot also have WAYMARK CACHES. There likely won't be a special icon for them (boo) but since WM's are virtual in design they serve well as Stages of a Multi-Cache.
After my 100th cache listing (I'm at 99 now and reserving #100 for an event) I am thinking of making a challenge cache that requires people to log visits to some of my waymarks in order to log the find. This would be a mystery caches since it is using "additional logging requirements".
I also have a plan to take people to 12 of my favourite waymarks as a multi-cache and when they find it I would write to them with a list of all of the waymarks they could also claim.
Jake39 has a geocache that uses 4 or so waymarks, I hope he chimes in with his expertise.
BQ