Hey stmb, not seen you around for a while
If your gonna go best bet is to stay in or near douglas becuase that is where you need to be for the best nights on the piss. It will be way to late to book B&B's or hotels there this late in the year, places on and around douglas go quick as hell . Not sure about campsite procedure.
From the UK side you need to get your ferry booked pretty early to if you want to get over on the 1st sat of the week, they get full up quick sharp - I presume it is probably the same from the irish side. Mind you I went in a transit van - so it might be easier to get on the ferries on a bike. Ah I just re-read your mail and your gonna fly over - Hey I wouldn't worry about not having a bike - i went without one and it was great fun, didn't miss not having a bike at all - I was way to pissed 90% of the time to be able to go anywhere near a bike anyway. Plus the fact that Mad sunday looked way to dangerous - to many mixed abilities on the road for my liking.
The IOM has a great tourist site which lists tons of campsites and hotels, I would grab some phone numbers of places in there (preferably close to douglas) and just ring around and see how you go.