I've managed to wangle a sample slip ring from a supplier who I've dealt with in the past, They are somewhat better than the Adafruit products:
Dedicated 100M Ethernet slip ring
Up to 250 rpm without loss
Splash proof to IP51
They won't sell individual units but I f there are enough people interested, I could set up a bulk purchase (10+) and I should be able to get the price well below $100ea.
I've been wondering about cable reels so thanks for those links, I'll be taking a closer look at those Markertec products.