Something that comes to mind is Discourse Signatures, so perhaps the user can link the cat file topic as their signature to appear at the bottom of posts? Maybe an plugin can “autofill” the signature if the user has only 1 ‘cat file’ topic? Unless I’m not understanding this correctly?
[Via a Theme Component] Either that, or a pre-filled composer template to include the link to the cat file if the user has 1 cat, otherwise a button in the composer toolbar to choose between cats, and add the relevant link accordingly? Again, I’m afraid that I may be misunderstanding, so feel free to correct me.