I have sometimes. I usually wait until there is no one looking - just after ordering. And just under the table cloth. I am pretty good at doing it without looking, except for dialling up. And as they look like pens, nobody seems to have batted an eyelid as far as I know. That way I am finished before the food arrives and no-one knows a thing.
I have had more trouble dealing with hypos - what are you eating that for, you will get fat? Or, that one is coming off something, look at her sweating. That is disgusting. Yes, I am coming off something, SUGAR!!! The staff of my local supermarket are used to seeing me eating as I go round, have come too finding an open box of sugar lumps in my hand at the checkout, or drunk 3 bottles of water when I am high, but they are very understanding, unless they are new.