I help run D of E at my gaff and as a MIAS level 3 instructor I’ve thought about putting it all together and doing mtb expeds.
My only concern (without having done a jot of research as yet) is that running under the Hampshire authority it is absolutely impossible to get a walking route approved that has more than the very smallest amount of road connecting footpaths (about 500m over the course of a day max). Once you are confined to bridleways only and with the larger distances covered by 2 wheels I can see the number of routes we could put together being very limited unless the rules were seriously changed for bikes instead of feet.
The other thing that would worry me is the increased potential for equipment faff – it’s hard enough to make sure they all have good boots and supply enough waterproofs/rucsacs etc but trying to cobble together some shocking bikes into expedition worthy condition AND keeping them going on the trip would be another level again.
As I say, not done the research and working through Hampshire who are renown for being H&S OTT which makes everything difficult maybe makes it harder for me than you, but it could be good fun.