Did some further research into the request to limit comments to logged-in users. It turns out that you can tune the anti-spam plugin to automatically delete comments below a certain spam threshold, and automatically publish those above another threshold. If this works, it will remove the need for any serious hacking to limit comments to logged-in users; the only real reason for this was to avoid spam. Set the values to -50 and +50 respectively; we'll see what results from this. If spam is still occasionally showing up, we can tweak again.