One thing is you need to make sure that whatever solenoid valves you get operate on the same voltage (12v) that that timer is putting out. Some operate on 24v. A better timer would have adjustable output to match voltage requirements.
Also, you could just buy an ac/dc plug in transformer and then hook up one of those timed relay devices to the solenoid valves. Lots of options.
Actually, you don't have a bad idea. I completely understand the reasoning behind wanting to be unconscious before nitrogen starts flowing.
Actually, I "toyed" with an idea like this for my CO setup. Planned on taking something to knock me out right after lighting coals. Then going into tent and passing out. Some time later an electric winch would pull buckets of coals on a cart into the tent, after I was knocked out, through a "tunnel" I constructed into the tent. The cart would have a arm on it that would close a door in the tunnel to keep CO from escaping. All very doable, but for me might not be necessary, if I can get CO level up to a very high level that will cause me to go unconscious after a few breaths, which shouldn't be an issue.