I agree with the comments about the electric lift system Parker fitted to the P275 & P285.
However, the P27 keel weighs about 100kg, the P275/285 about 320kg, and from the S850 details on this website its lifting keel weighs about 450kg. Hence a system that works well on a P27 may run into some problems with the heavier keels. I have replaced the hoisting system on my P275 with a 4:1 system that may be similar in concept to the one described by Martin Watson, but works with the weight of the P275/285 keel. There's a thread on this forum about it, search for "Parker 275 - New keel hoist system" this gives a lot of details and pictures, together with the sources I used for the various components.
I did visit an S28 called Aztec a while ago - she has an electric keel hoist system that the owner had developed and fitted. She is in the PSSA handbook with contact details for the owner - it may be worth you getting in touch with him.
I look forward to hearing how you get on, and the solution you arrive at.