A vague idea to get something similar what libquvi did.
Undocumented because it might change a lot, or even be removed. To give
an idea what it does, a Lua script could do the following:
-- type ID priority
mp.commandv("hook_add", "on_load", 0, 0)
mp.register_script_message("hook_run", function(param, param2)
-- param is "0", the user-chosen ID from the hook_add command
-- param2 is the magic value that has to be passed to finish
-- the hook
mp.resume_all()
-- do something, maybe set options that are reset on end:
mp.set_property("file-local-options/name", "value")
-- or change the URL that's being opened:
local url = mp.get_property("stream-open-filename")
mp.set_property("stream-open-filename", url .. ".png")
-- let the player (or the next script) continue
mp.commandv("hook_ack", param2)
end)