As far as I have seen there is not an "Official" rule list up. Some one please correct me if I am wrong.
However, The GMs/CMs have made comments in the past as far as a few known "exploits" but again nothing was really ever "official"
Correct, there is no "Rule Book" for siege.
Also, as posted in an earlier response in this thread. There are some reasons for concern about the volume of people that are using Fraps and Screen shots on people that are not using anything questionable.
I ran into this situation a few nights ago.
I had taken a tower outside of their castle. There members were all lower geared, with the highest being a 62 Warden, and a 62 Mage of about 28k hp. They continued to try and re-take the tower but I kept taking them out. They came at me with 3-8 members consistently. After about their first 5 attempts they started calling me a cheat, a hack, all kinds of wonderful names etc etc. All this only because they couldn't kill me. Then after a little while they gave up on the tower I was defending. Shortly afterwards the rest of our guild took down their front gate. I abandoned the tower and headed into their castle. I went straight to their processing factory and started attacking. The factory was at the far back, on the right. After I destroyed the factory I had 8 players on me, again calling names etc. etc. etc. I left the back area of their castle and notice they had rebuilt the gate and I had no others from my guild in the castle........So guess what the new accusation was?
Point is that depending on when a video is started, and how a screen shot is taken it could be made to look like someone is actually using exploits, even when they are not. If a video was started when the gate was down...it's all fair. If the video was started after the gate was rebuilt....I'm a hack. Videos and screen shots can be used as much as an exploit as the tactics in siege if so desired.