1) I created a new blog called Guestbook and added a single entry to it (the contents of the entry is the text you see above the "sign the guestbook" link on that page). Then, I made note of the Entry ID number MT assigned to it.
2) On my weblog, I hardcoded an OpenComments link referencing that Entry ID in my sidebar:
CODE
sign: <a href="javascript:OpenComments(136)">my guestbook</a><br />
So, when people click that link, they are commenting on something "outside" of the weblog.
Does that help at all?