As you've no doubt concluded by now, there's not really a way to do
this. It must be technically possible to override methods of the HTML
renderer to make this happen, but it wouldn't be practical.
I'm not sure what a good interface would be for more fine-grained
configuration. If it's worth it to you to look at how HTML renderer
might better support this, look for places in `scribble/html-render`
that use `external-root-url`, since that's what `--redirect-main` turns
into.
Matthew