Follow up - Yes, the ratcheting itself was slipping once friction on the D-ring was dialed in enough to hold the gear change.
The diagrams everyone posts for Suntour Barcons (as were posted in this thread) do not apply to the units I have. They are indeed the velobase link Jeff Willis posted.
The main screw on the shifter was being held in place by a tight fitting washer - a few hammers later and I had the unit disassembled. From there, I could see a bit of wear on the knurling of the ratchet and the engaging piece (looks like a tiny staircase), both made of plastic. I reconditioned their geometry with an X-acto and some jeweler's files I had around. I also reinforced the spring by well, jamming some plastic behind it so it had more force. It now seems like the unit holds gear, not thoroughly tested however.
I hope this helps anyone with the same problem. Buyer beware on these units. If the ratchet and engaging piece were both made of metal, I'd feel a LOT better.
Last edited by Adamwsanders; 09-11-18 at 09:29 AM.