I by no means am an expert or guarantee this will work in all cases or that it will be safe for all yoyos, but it’s an idea that may help answer some of your questions. Another product you might try especially for shallow recesses is “siliconized acrylic caulk.” I haven’t done an exhaustive search of the forums, but I have yet to see it show up in discussion. You can get it at a hardware store and you apply it in the same way as 100% flowable silicone but it’s considerably harder after it sets, so it’s best application is if you’re looking for a weaker response. It adheres well to both plastic and metal without getting torn out by binding. However, it can be pried out and peeled off fairly easily and comes off with little to no residue if it doesn’t work or you don’t like it. Just be careful and patient as it does dry fairly hard. You may need to pick at it with a needle to get it started. After it sets it’s just tacky enough to give you a little bit of response and any unevenness in its surface provides a weak starburst effect as well. You may also be able to shape it while it’s wet so that it has more response with a starburst effect.
I have successfully used it to replace the friction stickers on my Duncan Metal Drifter and Duncan Freehand Zero, both of which use rubber friction stickers. The recess was about 1 millimeter deep but the caulk holds well. The drifter now plays unresponsive and binds well without wearing out too fast (I don’t use the drifter all the time but it’s one of my beaters and I have not replaced the Drifter’s setup in over a year, but a more avid player will obviously need to replace it more frequently). The Freehand is still a bit grabby for my tastes, but the caulk still holds well and it’s less aggressive than the rubber pads.
I’ve also used it on a couple of YoyoJam yoyos, which were fairly well known for having some problems with chemical interactions with the plastic resulting in cracking in the hub. I have not noticed any problems with cracking or interactions with the plastic. I don’t know how it will do with loopers, though. It may only be useful with loopers with a well lubed responsive bearing and a narrower gap, and it may be able to be shaped so that it protrudes into the gap, but these are things that would need to be experimented with.
If you or someone else decides to try this, I have a couple of recommendations. First, I’d try it on a piece of scrap plastic first before using it on my favorite plastic yoyo, just to make sure it’s the right stuff and won’t become hard as a rock or deform the plastic. That will also give you the chance to work with it and get used to how to apply it. About 5 to 7 hours or so after you apply it, the excess and residue is easier to wipe off or scrape off with your fingernail without making a mess, then you let the rest of it set for the remaining 24 hours before you use the yoyo. It also shrinks considerably, so a deeper recess may need a couple of applications, but if the walls of the recess are only 1 mm high or less, one application should be sufficient. Again, I don’t guarantee that it will work for all yoyos, but if you’re feeling a little adventurous, it may work for some of the ones you had questions about like the ones that use friction stickers or slim pads.