In the README:

If no player is specified, it will use the first player it can find.

How is it determined which one is first? Is it possible to have some influence over that?

Alternatively, what do you think of adding a command-line parameter that specifies multiple player name, and the command would be sent to the first of these that can be found? (That way the list specifies a priority order.)

If this sounds good, could you point me to appropriate parts of the code where I could start making a patch for it?

(My use case is that I keep Rhythmbox open semi-permanently in the background for music and pause it when I want to listen to something else. That something else might be another player supported by playerctl. If so, I’d like commands to go to that player until it is closed.)