A tip from this post.
I don’t know if you (or most people) are aware of this, but beyond widgets, it has always been possible to control ANY plugin directly either though GP Script or via OSC using the SetParameter function or using the /handle/SetParam OSC message respectively.
For example, if you define a handle for the MIDI File Player (which you’d have to do even if there was an explicit action), then you can use the Set Plugin Parameter Value action to control any parameter exposed by the MIDI File Player.
The only reason I would consider this a workaround is because you have to calculate the desired track number value by the formula
(1.0/128) * (DesiredTrackNumber - 1)
