I had this issue recently when setting up a laptop for travelling. Apparently the custom plate is stored in the catalogue. I ended up just creating it from scratch, which is a pain. I can think of only three possible workarounds.
Firstly, creating a graphical ID plate outside of LR and use that. Secondly, copying an existing catalogue with its custom plate, rename it and delete its contents. Thirdly, creating an empty generic catalogue with the identity plate you want, and copy it for use with your clients.