That would be amazing! I have a garter bar set for the standard gauge machine, but recently purchased a midgauge machine (6.5 mm needle pitch) for which I would love to have a garter bar as well.
If I can help you in any way with the specs for the standard gauge needle stopper, let me know. I'm assuming that adjusting the stopper to the midgauge pitch wouldn't be a big deal. I'm only just beginning to understand the possibilities of 3D printing so I'm pretty stoked. :)