Thanks Gary.
I had already been pointed to the iFrame approach (by our very own JuggaloBrotha ) and it does seem to work. It was a little more difficult; the guestbook website seems to rewrite the url into a querystring and once inside the iFrame it no longer did that, so I had to get the real url and use that, but once I figured that out it seems to work.

Only problem is that the iFrame doesn't take up the whole available height in IE (it does in Chrome), which seems to be a common problem because it wants a container to fill to (and there is none). I'm just setting a height of 1000 pixels now and let the user scroll the guestbook, it's fine I guess but it'd be better to have the page stretch to the height of the guestbook so that there's only one scrollbar, instead of two...