I would not worry about thermal shock regards your crack as much as I would about heat cycles. In light of the problems that Cadillac and others have had with coolant/compression leaks, the sealer in the coolant idea seems prudent for your case.
If the cooling system stays full and in good order, there would seem to be little reason to worry about shocking the block with heat. Surface temperatures in the combustion temperatures have been published as high as 500 to 800F when the coolant is 195F. Higher loads would lead to higher surface temperatures. Of course it takes a while for the coolant to get warm, but if you start at -10F and open the throttle for full load, you have added a lot to the differential. The differential temp is what would cause the crack to open/progress. I would think that simply warming the engine by whatever method is available 50 or more degrees is about all that can be done beyond not loading the engine until it is actually up to temp. There was a semi owner/operator I used to know that insisted on running his truck below 10 mph for a couple of miles to warm it up. I think that is extreme but I am sure you get the idea.
Full synthetic oils have proven to be very helpful at low as well as high temperatures. We did a test one night when it went to about -25F some years back, the synthetic was the only oil that could be poured out of its plastic quart bottle. That means starting and oiling would be much improved over petroleum oils. A friend replaced petroleum oil in his race truck transfer case and eliminated oil related failures. I am not certain that the cost, which is coming down these days, will always justify switching but synthetic is something to consider.
