I don't think anyone is arguing for it being a rigid social rule. It's a pretty fluid social rule. Depending on the Noble/Templar, it'll be used against a Commoner, or ignored because it's beneath them.
I haven't really seen anyone do the second example, mostly the first example you used. It's a means to an end. A smart Noble or Templar will, well, be smart about it and use it to their advantage, just as they might use smoking spice to their advantage (which is illegal). A stupid Noble/Templar will not make friends fast be being a blunt shovel when an awl is needed.
Allanak gets a rap for being UNSUBTLE and BARBARIC, but i've seen more subtle/shhh/TulukRP in Allanak than I think people credit. The political system in general can appear on the surface rough and broad, but it's quite intricate. These are one of those intricacies, that requires imagination and nuance on the player's part to use to their advantage (or to the disadvantage of others).
Saying 'Well i'm going to wear silk because it feels better' is absolutely fine. Just don't get those silk underwear in a twist if some Noble/Templar decides to make your life worse for it, because they feel like it, and they don't like you. You don't need much of a reason as a Noble or Templar to make a Commoner's life miserable -- Doing something like this just gives them a flimsy reason to expand upon. It calls attention to yourself. If you're the kind of person where it doesn't really matter if you draw attention to yourself (you're the Aide of Lady Soandso, or a Merchant in Kadius), it probably won't matter. Just as wearing a hood in public draws attention to yourself. It isn't illegal to wear a hood, but it might draw suspicion, just as wearing silk might simply draw the attention of a nutty Fale Noble who has a vendetta against silk-wearing commoners.
I think we all pretty much agree that it isn't a hard and fast rule, nor should it be changed or codified that way. Like many things political, it is something that serves the rich and powerful, and oppresses the poor and unrepresented. Some Commoners with sway will wear silk without needing to think about it, unless if they run into their Noble's rival who happens to be drunk or just vindictive and makes them take off all their silk in public to humiliate them (Means to an End). This just happens to be one of the many things Nobles/Templars do to 'keep the common man/woman down', in my book.
It also doesn't really seem overpoliced, at least in the actual game. Sure we're discussing it on the GDB, but that doesn't reflect the actual game world in the slightest. I've rarely seen a Templar or Noble use this as a means to an end against a Commoner, at least I can't recall anything in public for a good long while. Saying it's overpoliced is like there are Noble Gestapo going around sniffing at everyone's clothing to see if it's made of silk. I don't think that is the case.
Also, saying that silk is "neither too expensive, nor unavailable to the average commoner. It's prominently displayed in the shops that are put right next to everything else, and for prices that almost any employed person can afford' doesn't discount the social phenomena surrounding silk and why it is considered a tool of oppression by the ruling caste. Black Americans might argue that sitting in the back of the bus, or the front of the bus, doesn't make a big difference, but to White Americans before the Civil Rights Movement, it did. To Commoners, even despite this sort of loose cultural understanding, they might think to themselves...What's the big deal, it's just silk? But the 'big deal' is purley that by whim, a Noble or a Templar can decide to make your life miserable if they choose to, silk or no silk.