Earlier this week I was brainstorming something like this.

There is a parallel between zapvertisements and livestream donations.

Donations with messages attached are a great way for fans to interact with creators. There is no formal contract, but fans know on good faith that the streamer will read and react to their message, especially if it has significant $ amount attached.

Now imagine a nostr client that enables creators to set a rate floor as you mention, and fans/advertisers pay that rate (or higher) to send "boosted" dms. 

Or even expanding upon that, fans could make offers to pay for follows, reposts, etc...