Just in case anyone is interested, I've built the prototype and added the controls needed for a proper test.
Works really well. Joystick and buttons are a-ok with no interference from spinner. Spinner games are great as expected, with the bottom row of buttons being more natural as primary inputs. And while obviously not ideal, rotary games are very playable using the spinner and bottom row of buttons in conjunction with the joystick.
The real advantage to this placement in my view is the wide range of playability within a compact cp footprint.
As always though, I'm sure many of you won't like the idea but just in case someone else is considering it i've thrown up the pics.