Simon, all of the NXEC trains have a separate secure guards van, bikes have to be booked in advance (free of charge) and can be done online at the same type as booking a passenger ticket. As an example I can travel return to London with bike for as little as £18.00!
There are 2 different trains, the diesel which I think can take 4 bikes and the electric which can take 6 bikes, I also think they can carry bents and tandems. In effect the guards van, certanily on the electric train, could carry 20+ bikes all stacked in there, as to whether you could speak to someone as to making this happen, well it would be worth asking. On a Friday night you have about 4 trains an hour going to Doncaster so even sticking to their policy you could transport around 20+ bikes and people.
A return from Hull would mean a possible change at Doncaster, these connecting trains have a policy of 2 bikes but not booked, it is at the guards discretion to take more. There are a limited number of direct services as well. If a number of people showed an interest and this became a problem I could look into alternative's.