To win a siege, means to capture the castle. However, Requiem's intention is not to win a siege but to solely go there for some PVP actions which was limited when they had a castle. As long as the Guild Leader doesn't imprint on the pillar, they will not become a castle owner again.
You can't imprint. As proven by Final.
Final signed up for Halland before they had captured Labonia.
This allowed them into the Halland siege even though they had a castle, though they were unable to capture the castle as they already had one. They were still able to participate and have fun.
The system is already in place. Just need to allow the sign up.