Selling mini crème brûlées
The question you are asking has no technological solution - you cannot put caramel on something wet and preventing from becoming wet. So you are looking at logistical solutions, and you have basically listed them already. 

For eating on premises, you keep the custard in the fridge and add the sugar and torch just before serving, as you mentioned. This is so well-suited and common, that I don't think anybody has had the need to develop any alternatives. 

For eating later at home, there are two options. One is as you said - you caramelize when selling, and the buyer has to live with the choice between eating it soon or eating it later without a snappy crust. The second is the one used by supermarkets and mentioned in comments - you package coarse dry brown sugar separately, and the customer has the choice between producing a good caramel crust at home (which is time-consuming and not everybody has the needed tools) or eating a custard with sprinkled brown sugar that has not been actually caramelized into a crust. 

A variation of the "package the sugar" along option is to package caramel along, either as a whole disc or ground into sugar-sized crystals. It doesn't require the customer to caramelize at home (so the tool and time drawbacks are eliminated), but it is clearly separate from the custard, making it less like real creme brulee. 

All two (or three) solutions fall short of the ideal, but as far as I am aware, you can't do better than that. You can just choose which negative is the most acceptable for you and your customers.