Verdict on the channels made of kitchen sponge: Success! Well, on the left foot. My left foot felt great for the entire session this afternoon. They really took the pressure off the top of that foot I could tell I hadn't placed them quite right on the right foot, but I was in a hurry to get on the ice and left it that way, and they didn't work as well. I think if I put them in the right place tomorrow, they should be as good as the left. If not, the problem is probably that the tongue is not wide enough on that skate to accommodate my foot and the sponges, since that instep is just a bit higher.
I'm going to try skating a few more times with the sponges, then I'm going to take rd350's advice and try to get some skate foam to use permanently.
For anyone lurking who wants to try this at home, all I did was get a regular cellulose kitchen sponge and cut it into strips about 3.5 inches long, 3/4 of an inch wide, and 1/3 of an inch high, and place them under the tongue on either side of my foot, leaving a channel in the middle. This allows the tongue to still (barely) meet the rest of the skate when I tie them. I placed them so they ran from the uppermost lace hole to the third from the bottom.