No Hellyer if theres rain, as their website states: "Rain cancels all track sessions." It would be really dangerous in the rain as the surface would be too slick.

Just to confirm you can't use bullhorns. You may also want to ensure you have bar plugs in your drops.

Can ride with clips and straps down there, I have. But unless you have stiff shoes you waste a lot of energy. Clipless is probably best, if you're used to them.