I made a quick search in the startpos files but it seems there is no zeal modifier.
The only trick that comes to mind is to mod a non vital building to lower zeal (Inn or Brothel might be useful, as the former isn't needed as soon as you build a castle and it gets far from the front lines and the latter is useful only if fully developed) and place is where needed.
I'm sure some expert modder will come with better tips but this should do the work...