Interesting method. I saw two sitting down eating today outside a restaurant. They were hotties but it didn't feel appropriate to just bowl in from literally walking along the street, dressed casually in my gym gear (I use an old baggy t shirt and shorts for the gym, not fancy form hugging clothes) and do a direct compliment approach. The waiter was also outside waiting another table so I feared confrontation for harassing the customers.
But what would a good opener be? "Hey, you two look like you are enjoying your meal. Mind if I pull up a chair and chat with you both" I just can't see it going down well. Maybe if I were more dressed up, but I am usually not due to practicalities.
Ι've approached girls like that sitting at a cafe or restaurant by themselves or with friends multiple times. I generally won't get inside the place, but if I am walking by and it has seats and tables outside, I will approach when I see a particularly hot girl.
Have I had much success with it? Not really, although that doesn't matter to me, for the simple reason that if I am going somewhere and see that girl sitting there, this is the only chance to talk to her.
I wouldn't just go and try to sit with them though, I generally give a direct compliment, explaining I saw them as I was walking by and wanted to come and meet them, and then if they are alone and they seem receptive I may invite myself to sit with them for a bit.
Again this is so out of the normal way people operate that it is pretty rare that it will lead somewhere, but still why not, you have nothing to lose, even if they think you are strange and the whole cafe looks at you who cares, you saw a woman you liked, you went and talked to her, no big deal.
Approaching girls in a place you are already sitting or ordering from is quite more effective in my experience. I had one of my biggest first successes in game, when I approached a girl sitting by herself in another table of a restaurant I was eating at.
Generally what I personally do because I am just not in the mood to interrupt my food, is as I get up before leaving I go and approach if there is a girl of interest that arrived earlier while I was already sitting there.
If she is there from the beginning or she sits very close to me depending on the place, I may do something more social and situational too.
I am surely not an expert at this social cafe approaching type though, mostly because I don't chill at cafes much, so I also haven't practiced it or found target places for it.
What I can say is that it needs calibration indeed, if I am at such a place I generally do very few approaches and try to make them as low-key as possible, unless I have decided I am leaving the place for the day, at which point I can go pretty direct and either stay if it bites hard or leave anyway.