There is no easy way to do this in FMOD Studio.
As a workaround, you could route your mono events into a group bus, then set the output of that bus to Surround 7.1. Then, route that group bus into a second group bus with a channel mix effect in its signal chain. Scope the sliders of that channel mix effect into a snapshot, and create automation on a parameter in that snapshot that at any given time mutes all channels but the one you want the event to play through, which it boosts.
Then you just need to set the value of the parameter of the snapshot to change which speaker the audio plays through.
Note that if you expect your game to use multiple different speaker formats, you will need to create a separate routing path for each.