Apparently official policy on hangers-on is that they are at the discretion of the Gurkhas, whose main priority is safety. At times more than four people have been allowed to walk as a team (although only four medals, meals, etc were provided). In practice, though, the walk is (almost entirely?) on public footpaths and as such there's nothing to stop anyone who wants to from walking alongside.
It's only a small leap of imagination to extend that to bikes. I expect they'd turn a blind eye to an accompanying cyclist so long as they were respectful and not disruptive.
As for train ride and cycling / camping... bring it on!