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.
Finished the circumnavigation in early February in Grenada. Have to work on a book project for the next several months so the boat will be waiting for next year.