I once heard that Captree stayed open late or all year, but can't verify that personally. It's nto exactly convenient to go over there.
With mine, when I've had some left in there, I dump in the no-tox, but not the pink. Use the blue or green or whatever it is, the -100 stuff. (WM in West Islip still had some this past weekend.) It's more concentrated so, when its diluted by the waste in the tank, you will get a lower freezing point than the pink would give you. At least, that's the way I figure it. I haven't had any problems with my tank freezing.
I don't see this as a problem from a disposal/environment perspective, since it is considered non-toxic, and it's going to the treatment plant once you get it pumped out next year anyway.