Traditionally I've seen these adapters primarily used to pass binaries for other architectures to QEMU and similar.

Years ago on FreeBSD I created a "Volkswagen mode" by using the similar `imgact_binmisc` kernel module to register a handler for binaries with the system's native ELF headers. It took a bit of hacking to make it all work with the native architecture, but when it was done, the handler would simply execute the binary, drop its return code, and return 0 instead - effectively making the system think that every command was "successful"

The system failed to boot when I finally got it all working (which was expected) but it was a fun adventure to do something so pointless and silly.

It would be a similarly clever place to maintain persistence and transparently inject bytecode or do other rude things on FreeBSD as well