Maybe there could be a waiting list for siege. Guilds that aren't sure on whether or not they will come will get registered as normal (if they get in) and then the guilds on the waiting list can report maybe half an hour before siege (I understand late arrivals, but half an hour is enough to get a rough value) how many people are available to siege/who. Yes this is a disadvantage, but better than not having siege.
If this is the case then the already registered guild can pick another guild on the waiting list to take their spot.
Multiple guilds (group perspective) that have been registered can select a group of guilds that can be randomly added in if a guild decides that they don't want to siege. So known proven point dropping guilds won't be brought into siege. This also would allow ourselves to police ourselves. GM/CMs can step in if people are being unfair
....unless it is TSF

cloaked can rot in
DL trying to perfect his strategy
^I dunno if there are enough guilds that actually siege for this to work so whatevs
Also, ppk should disband and then make mini-op guilds filled with newbies, then be mean to them and have them come to Reni or one of those pvp servers, apparently their newbs are worn out from rez stone campers
< edits original post for 15 minutes and makes post off-topic with additions, so anyone wanting to respond wait about that much time from the time I post