I have a furling main and want to know is it possible to reef going down wind? We were sailing down the Oregon coast at night and the conditions were too rough to turn into the wind to furl the sail. In the daylight maybe but with the swell and wind waves coming on from 2 different directions it wasn't happening. Should have reefer earlier before we had 30 knots and 6 to 8 foot seas.
Well, we have done so some times and it worked without a problem.
Two things we did before, though: our furling main exits on the port-side of the mast, so we took care the we were on port bow allowing the main to slide in more easily. Plus we tightened the main sheet so the main sail would not be pressed too strongly against the mast.
In general I would prefer hauling in going into the wind, though. Seems to put less stress on sail and furling system
I have reefed the main going downwind but not in 30kts. I always set the boom angle dead into wind and reefed gently - a foot at a time in this scenario. To be honest I think about reefing the main the minute the apparent wind hits 15kts. As I am sure you know on the 43DS it is easy at that speed but once it gets up to 18+ kts the effort is way more, unless you turn into wind.
An alternative is to drastically reduce the headsail to de-power the boat and go down wind on a broad reach if you are worried about an accidental jibe. Maybe not the most comfortable sail plan for 30kts from behind but having done a full sail downwind run in 35Kts (accidentally) I didn't find that comfortable in the slightest and the autopilot wasn't remotely interested in steering!