When I try to drag a region from a Smart Instrument track to an AudioUnit track (or any other), the drag process shows a “no” (slash) symbol. Copy and paste don’t do a damn thing either (whether copying between tracks in the track view, or copying notes from the region edit view into a different track’s region edit view).
It’s shit like this that repeatedly stops me using GarageBand on iOS. The tool is free, and yet it is actually pretty good overall for starting songs on the iPad (especially considering the expression available in Smart Instruments and the ability to use touch keyboards with different scales), but for these stupid frelling weird inconsistencies/limitations!