There was a NIP proposal for adding general contact info links which includes SimpleX. NIP-48 is for bridging content from other platforms, NIP-39 is for verifying identities of other platforms, which I suppose could be done for SimpleX addresses if there was a way to sign messages with the public keys. I think NIP-24 kind 0's are the most suitable for linking to SimpleX addresses as described here: https://github.com/retrnull/garnet/issues/8