Answer by Victor Eijkhout for Notation question: Adding half a beat to a bar?

You're overthinking this.

"and in 9/8, each eighth note gets a beat"

1. I could say "yes, and those beats are twice as fast" and be done with it. You get precisely the effect you want.
2. But really, music theory does not have beats that are independent of note values. That only happens in DAWs / sequencers. Normally the beginning of a piece indicates how many quarter or eighth notes there are per minute, and that keeps holding, even with time changes. So: do not indicate anything. The player will figure "eighth note here is eighth note there" and play it correctly.