I tried a cycle cape wrapped round (ICE Sprint NT) to baggy, I am going to trim it into a tabard shape and get Barbara to neaten the edges and put velcro straps on. Until then I use a Sprayway Goretex (intended for walking) Jacket with a double storm flap on top and have always been dry as a bone on the bottom with my Altura leggings.
When I rang Goretex they said no demand for a recumbent jacket, it is a pity there is no register of bent riders that we could contact to e-mail Goretex en masse. On a side note the same comment came from Broadribbs in Banbury when I asked with all the gorgeous machinery in there why they didn't sell ICE trikes, "no demand" he said, I then bent (sorry, sorry) his ears for half and hour explaining that yes, actually there was.