Now that we have site-specific custom close reasons, it got me thinking: why not make post notices customizable to each site as well? It's already known that the existing "citation needed" and "current event" post notices were primarily developed for use on Skeptics; sure, they are useful for some other sites, but not everywhere. Many other sites probably have their own particular use cases for text to be added as a post notice. In particular, I'm thinking of this proposed book recommendation banner on Physics.
I imagine this could be done with a UI partially similar to the custom close reasons, and perhaps the implementation could be partially similar as well (which would make it easier). What do people think - is this possible, and useful enough to justify the work on the part of SE devs?