It probably isn't as user friendly as you'd want, but you can reload the playlist by firing commands at a socket. See the "JSON IPC" section in the mpv man page, and specifically the load* commands. jo¹ and socat² are probably the simplest way to use it if you're not looking for heavy scripting.
The scriptability of mpv is really nice if you're the sort of person who likes building your perfect environment, and also a huge pit of addictive time sinks. </warning>
The scriptability of mpv is really nice if you're the sort of person who likes building your perfect environment, and also a huge pit of addictive time sinks. </warning>
¹ https://github.com/jpmens/jo
² http://www.dest-unreach.org/socat/