> is it possible to include a feature that would change the exterior style of animal houses?
Not AFAIK. I believe the house style is decided by the animal type (ie. "Prototype" in NPC_Tool)

>How hard would it be to be able to modify what the signs read?
The text on the small signposts is hard coded in the game, depending on your language.
(NOTE: when a Japanese visitor visits your town all your signs will read as Japanese)
The one exception is the bulletin board text (which you can edit in the game)

> $c839
> Armless invisible chair.
FWIW: It is on the item list as "not used" - $C838
NOTE: the item codes are normally multiples of 4 (ie. hex code end in "0", "4", "8" or "C")
The game uses the other numbers (the lower 2 bits) for the orientation of the furniture in your house
(ie. rotating it within the game will change the lower 2 bits). Also used for number of sheets of paper.
Whether Virus wants to support rotating items is up to him (it is rather obscure IMHO, it never got finished in the old AC:WW editor and few complained)