A clamp-on cable stop, already mentioned, will be the quickest, easiest, cheapest conversion. Not the sexiest.

I agree that it would be very cool project for somebody who knows CAD/CAM and has access to a 3d printer.

The "correct" conversion would be either:

1. Suntour made an adapter to do this, but they're not easy to find, generally don't come cheap, and I'm not sure it would fit this particular boss. Miyata mostly spec'd Shimano, and I think Shimano's top-mount boss was larger than Suntour's. Maybe Shimano made an adapter as well?

-or-

2. You could find yourself a set of top-mount shifters, remove the levers and attach cable stops to the mounting block. You'd need flat-backed stops, not radiused ones, or you'd need to file down a set of radiused ones. And you don't want to use the self-trimming Suntour Symmetric shifters for this b/c the mounting block won't fit std cable stops. Also, see #1, not sure a Suntour shifter will fit this boss.

Also, check the archives here. I found a bunch of BF hits when I googled, so the topic pops up every now and then.