In simple terms, a big nope. Different freewheel cog spacing, different derailleur geometry, different indexing mechanisms. Getting Suntour indexing to work properly and reliably in a production setting was a challenge. Getting mixed systems to work on an individual level is pure luck. From a design/engineering perspective, it should not work well. If it does, most excellent for you. If it doesn't, well, it shouldn't.
At one point, we sent out Go/No-Go steel gauges to dealers to troubleshoot Suntour shifting problems. They had to check for correct inter-cog spacing, which required three different thicknesses of steel slabs, and microspacers. What we were saying, essentially, was that cog spacing being just a few tenths of a mm out of spec was enough to cause inconsistent shifting. That is not a forgiving system that should do well when mixing critical components from other mfrs.
Since the cog spacing was different, Shimano didn't/couldn't design their indexing components to work well with Suntour equipment. And as far as aftermarket sales go, better to force users to buy shift levers, freewheels and rders to convert.
This and your following post are fascinating! Thank you for expanding on all of it. I just picked up a beautiful dark blue metallic 1985 Cannondale with a mix of C&V but not-original parts (close to '85-90 era though) and the shift levers are the Sprint 9000/Superbe Pro (can't tell the difference!) with the two indexing settings and a friction--working a Sprint 9000 RD which I will have to try to work over the 7s cogset. Haven't tried yet since I didn't test ride (a killer price on a pretty bike with a new Brooks Swift saddle--no problem).
On an '89 C'dale R700, I picked it up with full that-year GXP. Hidden spring in the calipers (really cool), "7-speed" downtube shifter that had 7 clicks, not the normal 6 clicks for a 7-speed (# of indexed speeds minus 1 = # of clicks in my experience), and thus indexed 8 speeds. Indexed an 8-speed, 5.0mm-spaced cassette well. It took a little finessing on the tension,
and going back from a Hyperglide-cogged 'custom' cassette to a Uniglide-cogged setup as the RD would hop back and forth on the 3rd and 4th cog automatically. I presumed it was because of the ramping that encouraged shifting, but in the GXP's case, it was too much encouragement. Going back to an older cog profile, to me, made sense as Suntour's freewheels (to my looking and knowledge) or their spec'ing had pretty bolt straight tooth profiles in both axes (yes, with some chamfering/shaping)...that and Hyperglide was a year away. A Shimano 5800 rear wheel with a 7s-spaced 8s Uniglide cassette shifted by an old Suntour system. Good times! Still really like the ratcheting FD shifter.