Why not use a rap ring on a V-thread?
Mostly because it is not needed. The original post from Richard (yo - big up) claimed that pulling ropes through sling material would weaken the sling and render it unsafe. Both the empirical evidence of thousands of climbers, as well as pull-tests by professional gear testers, have shown that this is not the case.
Visually inspect the sling. Slings that look fine are fine. Damage from heat will not be hidden, it will be an obvious melt mark.
This does not often happen to v-thread slings. That v-threads are used in winter is significant. Cold air, water, and materials (sling/rope) reduce friction and dissipate heat. When was the last time that you rapped a rope in winter and found your belay device too hot to handle? When it does get hot in summer, notice that that is still not enough heat to melt you rope.
Slings can melt from friction. Repeated pulling can do this, it just doen not happen much on slings used for short periods in winter. It is more of a problem with permanent fixed rap starions used year-round. I fully support rings and quick-links on fixed rap stations like trees. I just think it is littering to add them the v-threads.
Most important is that the rope must not be weighted when it is pulled. It easy to melt sling material if, for instance, you take a lead fall on a rope that is threaded into a sling. But that is a vastly different force and load model than pulling a rap rope.
Slings are pretty darn strong. Several independant organizations (not manufacturers) have pull-tested sunbleached ratty-ended scraps that have been fixed for several seasons at altitude, and found that they broke at or above spec. Heartening isn't it. The thing I worry more about is a poorly tied knot.
Glad you liked the article though.
ps, I will have to search through a mess of old web links to get you the references (and I will try to do this soon), but I have read several articles from Black Diamond's gear testers on this particular subject (check out Fish's website if you want to dig a bit).