I have the same issue, and I personally think it's stupid (as in, in what way is talking to a stranger in public weird?). Thinking it's stupid doesn't make it much easier to overcome my own inhibitions about it, but it is somewhere to start.
I think tact is the key. Interrupt as politely as you can, and gauge their reactions when you do. If it is clear they are not interested in your input, then turn around and leave them alone. No harm, no foul. If you have something to contribute, though, and the individuals weren't specifically seeking a private conversation, then they will probably be interested in what you have to contribute.
This reminds me of a recent episode of the Ricky Gervais Show (basically Ricky and his friend make fun of another friend of theirs the whole time, funny but it gets old), where one of the hosts went swimming, noticed the guy in the lane next to him had an excellent front crawl (which the host has always struggled with) and asked the guy if he could give him some tips. Ricky's response was "Oh god, you didn't! Why would you do that?!" My thought the whole time was why in the world is that wrong? If the guy isn't interested he'll say no, and that will be it. If he is willing to help out a fellow swimmer then he will, and they may become friends over it. Where is the loss for anybody there?
I have a half dozen friends now that I wouldn't have if I hadn't done something very similar a couple years ago, at a swimming pool too, no less. I simply started talking to the lifeguard before and I after I swam. Not quite as out of the blue as the Gervais Show co-host, but it was similar.
Still, some people find it rude. I don't for the life of me understand why, except for when they are clearly having (or are attempting to have) a private conversation, or talking about a personal. Otherwise, where is the harm? And really, the risk for me personally is extremely low. So some stranger thinks I'm odd, so what? Most people are odd in some way, friendliness is far from the worst odd trait you could have.
Strangers are a potential threat. So when a stranger comes up to you and initiates a conversation, there's some reason to be on your guard.
This is combined with basic etiquette. If someone makes a small request, it is considered rude to refuse. The problem here is that creepy weird dangerous strangers can take advantage of this fact by making a small request, which then makes you feel obligated to comply. So now a complete stranger, who may be dangerous, has ensnared you. You're now doing something that he asked, instead of something that you want to do. A...
I am beginning to suspect that it is surprisingly common for intelligent, competent adults to somehow make it through the world for a few decades while missing some ordinary skill, like mailing a physical letter, folding a fitted sheet, depositing a check, or reading a bus schedule. Since these tasks are often presented atomically - or, worse, embedded implicitly into other instructions - and it is often possible to get around the need for them, this ignorance is not self-correcting. One can Google "how to deposit a check" and similar phrases, but the sorts of instructions that crop up are often misleading, rely on entangled and potentially similarly-deficient knowledge to be understandable, or are not so much instructions as they are tips and tricks and warnings for people who already know the basic procedure. Asking other people is more effective because they can respond to requests for clarification (and physically pointing at stuff is useful too), but embarrassing, since lacking these skills as an adult is stigmatized. (They are rarely even considered skills by people who have had them for a while.)
This seems like a bad situation. And - if I am correct and gaps like these are common - then it is something of a collective action problem to handle gap-filling without undue social drama. Supposedly, we're good at collective action problems, us rationalists, right? So I propose a thread for the purpose here, with the stipulation that all replies to gap announcements are to be constructive attempts at conveying the relevant procedural knowledge. No asking "how did you manage to be X years old without knowing that?" - if the gap-haver wishes to volunteer the information, that is fine, but asking is to be considered poor form.
(And yes, I have one. It's this: how in the world do people go about the supposedly atomic action of investing in the stock market? Here I am, sitting at my computer, and suppose I want a share of Apple - there isn't a button that says "Buy Our Stock" on their website. There goes my one idea. Where do I go and what do I do there?)