I love seeing a good smackdown, and that one was classic, Endo. The existing mechanics cover he siege engine issues quite well, as any engines are assumed to be destroyed if the attacking army retreats and abandoned or used for firewood/reconstruction if they win.
The only thing the existing system does not cover in the abstract are ballistas. Those would be nice, but they'd also probably slow any army down to a crawl, and we don't need all the extra graphical fluff that showing the siege engines would entail.
Edi