For something like that to work, you would need to be able to select a relay to read from that won't accept posts mentioning you without the user paying first.
You don't have that option with Primal, because you can't configure which relays Primal reads from. It only reads from their caching server.
#Primal is just a full stack client; their backend gets your posts from your relays.
Yes, Primal's caching server aggregates my notes from the relays I have set up to write to. However, it does not allow me to read directly from the relays I want to read from. I can only read from Primal's caching server, which pulls in notes of other users from the relays they wrote to.
Therefore, any web-of-trust filtering on my read relays, or payment requirements for someone to post to them, are to no avail. Their note will be aggregated by Primal's caching server from whatever relay they post to, regardless of the ones I want to read from, and displayed to me.
Indeed, you can't even set which relays are read vs write in Primal, because they don't let you read from relays. They only let you write to them and read from the caching server which aggregates notes from all known relays.
