There was riders wearing waterproof jackets. seen a few riders wear these around the shows. May be the case of looking round the tack shops and the internet.
there's a company that makes racing colours that makes a very lightweight waterproof jacket in your colours. can't remember the name at the moment, have a google!
Looked like Matthew Wright was wearing one - I wore my lightweight mountain horse jacket for XC once as it was tipping down. Kept me a bit drier than I would have been !
TC & PF had them on at Firle last weekend. Tina's was a Musto one (think she's still sponsored by them) and Pips' looked like the Team GBR/Toggi one which is on the Derby House website.