Is there a plan for this or something similar?
We're open to integrations with guestbook apps, but don't currently have any. I know a lot of them can integrate via iCal which we do support. We also have an open API if they're looking to do an integration.
If you've got a contact at a guestbook app that wants to integrate with us, feel free to send them over to firstname.lastname@example.org and we'll discuss.