I have been working on porting this myself (and making quite a mess I must say

).
I think what needs to happen is to create new templates for any "calendar" forums, and load those templates from newpost, addpost, etc.. The templates would be something like addpost_moderated.calendar.tmpl, newpost.calendar.tmpl, etc. They would be basically the same as their counterparts, but they would contain all of the modified HTML that the calendar mod incorporates.
Then inside of the newpost.php, addpost.php, etc you would have some logic that says if $Calendar == Y then load the proper template.
Am I on the right track here?