I know I wouldn't mind seeing a damagable house for Guild wars, It would almost need to be a custom house, having all houses attackable might be a bad thing unless it was say fel rules. Another thing to consider for having attackable houses is the fact that the owner of the house should be loged in for it to be attackable. Otherwise, you log in as a homeowner and find yourself in the middle of a field with no sign of your possessions, or your house.
As Osi has it set up now, RP Guild leaders need to talk to one another in secret, saying ok my guild will attack your town some time tonight. and the defencing guild would tell its peoples that the "our scouts report that the orcish horde is gathering and an attack may be expected" With that in mind, a damagedable/non-damagable switch would prob be recomended.
Needless to say I've thought about seige weapons quite a bit, and the fact that Richard Garriot belived that everything in the game should have a purpose, with catapults and balistas (both having anamation graphics using Pandoras box) its safe to say OSI has thought about it too. But with the proplems listed above that makes it hard.