I seriously doubt that anyone would be turned down. Mersey Roads struggle to run the events and some are trying to stop it from being run. More riders means that there is less justification for not running it. The more the merrier. If you've ever ridden a 24, you'll probably have an idea how much of a big deal it is to run a 24hour TT.
I entered my first 24, the Mersey Roads National Championship, with a blank entry form. The North Road 24 was also ran that year, so considering I was a novice with no obvious form entering a National Championship event, I'd say they'd let almost anyone ride. It's not uncommon for riders to not even manage 300 miles, so don't be put off entering. In fact, you'd probably be doing us a favour by entering.
There will be a 24 hour in Southern England in 2011 as well as the Mersey Roads 24. I think it will be organised by the Brighton Mitre. Unless something bad has happened to prevent it being run and I don't know about it.