Chuck,
You will only get a temporary seal with the poly over the drain. Pretty quickly water will leak in and seek the level of the pool water. If the 35' run is above the pool water level it will remain dry. How about installing a ball valve on the line where you hook the shop vac? When you get the line blown dry then just close the valve. Problem I can see is these valves are not necessarily fully air tight under air pressure and over the many months of Winter may leak a little. They should be a good air seal but ya never know. Of course over the Winter you could blow the line once in a while. Just my opinion, and I don't have a main drain, but I believe as long as water is in a verticle pipe heading to the drain, let it freeze as the frost depth increases. It will freeze and plug at the top then progressively work the ice plug lower with the bottom end always being open to relieve expansion pressure. Pipe shouldn't break. I can see a scenario where you get a long hard freeze, then some time where the surface temperatures climb above freezing thus melting the top end of the verticle pipe, then re-freezing. Two ice blocks...surface and one one deeper. As the center freezes...poof. I certainly welcome comments on this statement.
Al