Yep, that's about it. I used the stuff from the link several posts back, which is a two-part resin each with a similar (very thick) consistency for both parts. Because I wasn't sure how much it would stick to things, I put a disposable zip-top bag into a cup, poured in what apparently was equal portions from each bottle (it's hard to measure because of how thick it is), stirred the two parts together with a broken half of a bamboo skewer, and poured it over the top of the plug. I didn't take a video (my hands were busy with the cup and the skewer to help scrape as much as I could get out of the cup) but it's pretty simple - try to pour directly onto the tip and make sure it flows evenly down all sides. I used the skewers to help make sure it got onto the overhang parts, and to try to keep the puddle from going off the edge of the glass. I didn't take any measurements either before or after pouring but I suppose I could try to at least get a measurement around the fattest part of the plug to see how much thickness I got in that exact spot. My guess is that it's pretty thin at the top where the color of the Ninjaflex shows through, and that it will be far thicker at the base than it is at the top.
For what it's worth, I did all of this on the smooth side of my glass build plate for the one printer. Again, not knowing how well the silicone would bond to things, I used a little purple glue stick as a barrier; although I ended up not coating enough of the glass with glue stick to account for how the silicone puddled next to the plug, it came off pretty well on its own...a few bits here and there were left behind but a wet paper towel to remove the remaining glue stick and the part scraper that came with the one printer were sufficient to get the glass completely clean.
And I was able to play with it a bit in the shower today. What I learned is that the silicone coating is super smooth, it did a phenomenal job at covering up the layer lines even with the thinner bit towards the top. However, I don't think 10% infill was sufficient - it deformed enough that it just squished when I sat on it. Granted, it's been a while since I've been able to get a good stretch going and I may give it another go with a little more warm-up, but I think the Mk2 version of this experiment will involve 15-20% infill and raising the area of dense infill from just the start of the neck and down into the base, up to maybe halfway between the top of the neck and the widest point. I don't want to take the dense infill much higher than that because then it's not going to be as comfortable, but I also don't want the plug to just crumple rather than going in smoothly. Perhaps I'll leave the 50% infill section through the top of the neck alone, go 35% from the neck to halfway to the widest section, and then 20% from there until dialing it up to 100% at the very tip (for support purposes).
I'll probably also try out the second plug before I cover it, because I maybe used a third of the silicone on this first attempt. It says it has a limited shelf life and I don't want to buy a whole second kit for only one use...some of the other files I've found (mostly in the Naughty section of Cults3D.com) have some unfortunate overhangs and/or gaps that will be a challenge to print in a flexible filament.