I second the use of bearing grease on the tap, but if you have BoeLube, that works even better. I would combine that with the idea of blowing air into the other plug hole as Mike said. If you don't feel like welding, I'd recommend using a compression tester and plug low pressure air onto the fitting.
Knowing that you'll be looking down the hole while crud is coming out, I would not use more than 30 psi, and I second the use of safety glasses plus a face shield.
- RV10, Build in progress.
- RV12, N975G, "The Commuter"
- Several others that are now just great memories for me.