No Wayman <iarchivedmywholelife@gmail.com> writes:

> It makes this sort of customization very simple

Famous last words. I forgot the agenda relies on text-properties 
for much of its functionality.
I've attached a patch that addresses this and also removes the 
`save-excursion` around the funcall to the custom insertion 
function.
I've noted in the documentation that if the function is moving 
point, it is responsible for making sure it ends up on the line 
before the next habit.