A programmer advised not to try changing the size of the AI grid, since that will break saves (the coordinates are stored relative to the AI grid, so if it changes they may no longer match).
I realise you may be trying to add an area to avoid content added by various other mods, but there is a reason everyone stakes out an existing location to use.
Unfortunately it is not possible to generate a new AI grid partially; one of the scripters thought it might help if you reload the level without reloading Story before regenerating, but wasn't sure if that would break anything else.
The drawbridge is a special case, and the people who responded didn't recall the details of how it was done.
