Thanks everyone, these are some great ideas. Really helps me organize.
Based on your ideas, how is this for a plan? :
1) I probably won't be able to get to a store for silicone for a day or two, so I will leave the tank / filter / cycle, ammonia, the works going until then. Hopefully, nothing will explode in the meantime.
2) Trip to the store for silicone + new substrate + maybe a ten gallon (or less?) tank. Hey, btw -- how small could my hospital tank be? My regular tank is 17 gallons and I plan on only 2 or 3 small fish in it. Could I do a hospital tank of like 5 gallons?
3) Fix myself a nice cocktail
4) Tear down the tank, emptying substrate, rocks, plants into new small tank
, along with filter. Keep cycling in there, ammonia and all, filter + heater running
5) Fix leak, let tank dry. Put NEW substrate in the tank, put rocks, plants and filter back.