I would talk to other clubs who sponsor rides and see what they do for insurance. There may be specific brokers you can go to. I am sure there are single event policies you can get for this sort of thing. Then you could estimate the number of riders you will have and pass the cost along in the entry.
It may up your costs, but I believe it would be worth it. Even if you have a well written waiver and you ultimately don't have any liability, if you have a claim or get sued you still need to defend the case, and the insurance would cover you for these costs. Sometimes the defense costs can be more than the claim is ultimately worth.
I am a defense attorney, and do a lot of work in this area - waivers, etc. (although the actual procurement of the insurance is a bit beyond me). If you want to email me off line I would be happy to take a look at your waiver form and maybe give you some suggestions.
Sarah