I am using your plugin and it seems to work properly. However, I would like to
go a step further when tagging a build. I was expecting the tag to also peg all
svn:externals automatically which the plugin apparently doesn't.

Since Hudson knows this information, I figured it should be feasible to peg
them.

Note: We're careful about keeping flat svn:externals, we don't have externals
of externals therefore pegging a tagged trunk seems natural and doesn't involve
pegging the externals' children.