What kind of soda are you planning on doing in the keg? The potassium metabisulfite would help, but it all depends on ingredients and sanitation practices. If you don't want to mess with preservatives, you could hot fill it. You don't need a pressure canner because the pH should be below 4.6 anyway. I think carbonic acid on it's own is 4.3.
Clean and sanitize your keg. Fill it at 190° or greater. Make sure you have silicone o-rings because they withstand the heat a bit better. Seal and put it on pressure right away, because as it cools, it's going to pull a vacuum and you'll lose your seal on the lid without positive pressure. Flip it upside down for about 5 minutes so the liquid comes in contact with all the inner surfaces of the keg. Then set the reg to carbonation pressure and forget about it in a cool spot for about a week. (takes about 12-16 hours to cool to room temperature, so factor that in to however long it normally takes you to carbonate from there)
This isn't perfect, because there is a possibility of introducing contaminants via your CO2 source, but it's better than nothing. With proper sanitation techniques, I've had unrefrigerated kegs of soda in the basement for longer than I'd care to admit without a problem.