I would love this.
We do PK wars in our guild castle sometimes, but a practice siege would be soooo much better.
It would be stupidly fun, first of all, and it would be helpful to all for training purposes.
I wonder if they could use the battlefield concept to do this, like Tyrefen Mountain Range on acid, using the siege map instead. You'd split you guild up into groups, limited to 36 people per team just like siege is. I don't know crap about programming, and I don't know if the battlefield server could take the load, but surely if it can handle all those real sieges all at once it could take the burden of individuals guilds at different times.
We could even do Guild wars that way, as an option instead of open-world pvp. I would pay dias to do that, forget gold.