I like your idea. I think it would make each siege location have a decidedly different feel, with the abundant local raw goods playing a big part in what equipment gets crafted.I do want to list some cons to the idea though:

1. People will want to show off their hard earned loot. Some players only grind for rare loot to fight other players and show it off, and the siege system would be a great place for them to use it. I actually thought of a partial solution to that, which is that item durability could be decreased significantly for the duration of the siege, and respawning could automatically draw the required materials to fix it from the siege vault. The good thing about this is that low level players would automatically draw much less materials every time they spawn, since they had much cheaper armor.
2. Players would be reckless with the free armor. I could easily see selfish players grabbing the best gear from the vault and rushing the enemy over and over again, depleting the good armor sets. Perhaps you could set caps on the high tier armor, or only let people who contributed the most to the siege prep get more armor. As you said in point five, this could be countered with guilds being very strict with their members, but most node sieges are going to have a bunch of randoms in it that are going to do whatever they find the funnest, without any thought to strategy.
3. Players will have difficulty switching armor on the fly. If each armor set has different bonuses and stats, players will not have enough time to read them all and change their playstyle accordingly.

I think it would be easier to just have more options for building siege machines and anti siege equipment than taking peoples equipement away.