Thanks for the calculation, my trysail is about 90 sq ft, so about 20% of the main. I used your formula and got just under 1400 lbs at 60 knots and just under 2000 lbs at 70 knots. So considering that I would use it as a turning block that would make the load somewhere between 2800 and 4000 lbs. The 5000 lbs is the safe working load of the block - with the breaking strength at twice that I would assume. Not sure what the 8500 lbs block would be used for - what a beast it would be.
As for hooking up the sheets, I would have to use the cleats since I don't have a slotted toe rail - just the fancy and not very functional teak toe rail (good for keeping your feet onboard though). I have a couple of really big snatch blogs but if the shite hits the fan I have them tasked for the parachute anchor set up like the Pardeys suggest with a bridle and I might well heave-to with the trysail still up and sheeted in tightish.
Heading back to Lake Ontario for this summer. Relatively few stops along the way from Grenada. Martinique, Guadeloupe, St. Martin (must have something to do with the French food), then Bermuda, New England and up the Hudson/Erie Canal. We were going to go via Newfoundland and Labrador but June remembered that one of the kids is getting married this summer - details, details!