You can't offer an oath in the normal way. The only way that I've seen it work is that when they become ruler, they "already" are a noble belonging the region of the capitol. They have an oath to the capitol, but without any tax share. This can be rectified by offering to give them a tax share as one would a new noble to the realm that is "belonging to your region".
Interesting. This must be extremely rare however, has anyone else been in this situation who is able to confirm this?
