To assist the prevention of parts going overboard:
- As already suggested by Ross, cut a round hole, just a bit larger than the winch drum, in the bottom of a suitably sized but shallow box. Then you can begin to disasemble the winch inside the box. But be sure that the box is secure on the surface around the winch .... you don't the whole box tipping over. I've used some duct tape between the box and the boat to help prevent this.
- Tie a very large towel or tarp up on the lifelines adjacent to the winch, then drape it across the deck towards the winch. Anything dropped has a good chance of being caught before going overboard.
- Because of the grease, parts can stick together and then unexpectedly fall off. Be mindful of this. In particular, the main roller bearings between the center spindle and the drum can stick to the drum as you lift it off, then drop out of it.
You mention "25 years". If they are Barient winches, check this web-link which has pdf's of parts the original illustrations and maintenence procedures for all the Barient winches. And also some Barlow and Lewmar winches. Many of the Barient winches are extremely difficult to completely disassemble in place because a spring retaining clip which is located at the very bottom of the center mainshaft is is very difficult to remove and get back on. But the Barient instructions say that lubricating the mainshaft in place with oil is OK. Notwithstanding, 90% of the winch can be disassembled in place.
http://www.cncphotoalbum.com/indexes/winches.htm