Hmmm difficult. Npubs have a very complex checksum that is never used and very difficult for solid to compute.  It also is the wrong check sum (segwit, instead of taproot, doh!).  Npub would serve zero purpose here as we already have a nostr predicate.  Likely hex public key which is the canonical hex pubkey as better.  Npub is a mess of technical debt but is likely to stick around.