The third paragraph [in Poll and Ping Modes with Pairwise Identifiers] specifies in rather weak language how the client shall demonstrate possession of the respective private keys. Moreover, the check is deferred to the actual use of the CIBA functions. In contrast, in case of standard OIDC the check whether a redirect_uri belongs to the authorized destinations for certain PPIDs is checked at registration time. Deferring the check to the CIBA use puts the respective RP record in kind of a middle state.
Have you considered to let the dynamic registration function of the OP perform the check? One could use one of the methods cited in the spec (mTLS or private_key_jwt) to conduct the proof. Such an approach would allow to conduct all the checks necessary in one place and a single action and either accept or refuse the registration.