I would recommend the co2 method as well. You can buy a kit that threads into the drum in place of the nipple that uses co2 cartridges to blow obstructions out of the barrel. The grease gun idea sounds feasible as well. I hadn't heard of that approach before.
JW- close on the press fit. They are threaded, but the plug is oversized. They heat the breech up to expand it, thread in the plug and index it. Then when the barrel cools and shrinks it sweats in the plug or press fits the threads in other words. The drum does indeed thread through into the breech plug as you stated. So the drum or touch hole liner on flintlocks has to be removed first. There is a procedure to removing these breech plugs, but since it requires heat it is not an option in this situation. Since there is more than likely a charge of powder in there somewhere . If you go with co2, remember that the obstruction will come out with great force if it does indeed come out, so be prepared for that. Here's a cva breech plug from a kentucky flintlock. Josh