My friend George passed on this suggestion via email, asking me to post it for him:
"When turning rather rustic wood (lots of bark, cracks and knotholes) I often will encounter a major void, especially when the heartwoods begin to dry and shrink. The quickest way to fill these is actually with a piece of natural cork, like the ones that come from a wine bottle. It's flexible, fits snugly, and will accept both a stain and a finish. Last week I had a knothole fly out, and in two minutes I had it patched and ready to continue without waiting for glue to dry. Truth is, it looks like the original knot."