It seems easier to just use ligess (with zap support) and add private=true 😂
It seems most LN address server didn't explicitly set private route hints or have configuration for that.
I found that https://github.com/bumi/lnme has config for that. I already use it for my private LN address outside of nostr. But, i haven't decent skill in Go to patch it to make NIP-57 compatible 😅
Discussion
Yup, only need NIP-57 and voila ready for action 😂