^Whilst I see your argument for the increased force on the biker, I don't think it's quite right. Disclaimer: I know these are simplified models, they're just an example.
If you look at this picture, it is quite clear that the 'm' term cancels:
mgsinΘ=component of force down slope
μmgcosΘ=max. frictional force
I can't see any incorrect statement in that. I know it doesn't include air-resistance etc, but it's a simple model.
This model explains the problem using moments. Here the 'mg' term should read force. I understand that the force would be distributed between the wheels and that the force on the front fork would not be 'mg'.
This model includes a term which is dependent on the mass. As I'm sure you'll know, the moment would act as a turning force, therefore a larger turning force would equal a larger gain in speed (in a roundabout sort of way).
I hope this is right, it seems pretty logical, but I'm not afraid to admit I'm wrong, so if anyone can correct me, then feel free.
If the coaster is not powered, I don't understand why you'd need to centre the mass?
I mean that the train will still be able to crest the hills with all the mass in the back of the train (ie. four fat riders in the last car) or however the mass is distributed throughout the train.