[crossfire] Modifying Multi-Square Objects

[Preston Crow]

I'm having trouble working with a multi-square arch.  I'm trying to 
modify it so that one square that is normally blocked on the roof of a 
building is passable.  In the arch, it's split up into four separate 
objects, for example:

Object guild
...
end
More
Object guild_2
...
move_block all
x 1
end
More
...

It's this second square that I want to modify, and I can't find a good 
way of doing it in the map like I can with single-square arches.

To just remove the move_block from the second square, I tried using a 
different building's arch and changing the face to the one I wanted, but 
the face substitution doesn't seem to work.  Ideally I would like to be 
able to control other aspects like shifting the hp/sp fields by one to 
change where you enter (or even something really weird in some cases).

If I could access the four parts of the image separately, then I could 
just use four one-square building arches with the face adjusted to look 
like one big building and get exactly what I want, but there doesn't 
seem to be a way to grab just one square of an image.

Unless there's a trick I'm missing, the only thing I can think of is to 
create a custom arch just for this one building, and I would really 
prefer to keep these changes in one place if possible.

Is there a trick here I'm missing?