If you do save deletion with the autosave that triggers when you walk into the area in A Red Letter Day to get the crowbar (or, preferably, skip the crowbar), the door behind you disappears. If you walk through it, hit that loading zone, the “A Red Letter Day” text comes up, and then walk back out the door and load, the Combine foot troops’ AI stays stuck in the “Nothing interesting here, move along” mode that they are in at the start of the game. Shooting them doesn’t snap them out of it, but I’m pretty sure using save deletion again would. (I haven’t tried any other methods to get this to work, so walking back and forth through that doorway probably isn’t needed, but there are barrels there if you are having trouble wallclimbing with a pallet.)
Don’t expect me to stick around for long, I’m scared of forums just a random viewer who found something. Anyway, have a nice day.
Edit: This only works if you do crowbar skip. Not something any of you should be worried about, but hey.
Edit2: This also breaks if you don’t do the first wallclimb in Route Kanal. I should probably explain how I think this works: all the triggers for “now the Combine start shooting at you” are skipped. For instance, the part where you break through your first planks with the crowbar and the music starts, and when you jump across the train and up the ladder in that spot in Route Kanal.