Jeff,
Great idea for the placement. This is on our Wishlist as Set Transistion Altitude. The ideal is to have the Set Transistion Alt option on the OPT menu on Flight Plan. Then the VST instrument would pop to guide you to that Transistion Alt.
I think you may be more asking for the "Hold Altitude" option or "Target Altitude" that is not assigned to an actual wyapoint. Example, simply tap the VST instrument and set an Target Altitude. Rather than actually assigning an altitude for a waypoint on the flight plan. If so, I like this option and have added it to that Wishlist Transistion Altitude req.
Also, maybe tap the Altitude instrument for a "Target Altitude - VST" option), since the VST instrument is not always visible.