Electronic – What does this fabrication rule do

I understand what you want, but it doesn't make sense: the drill diameter is determined by the hole diameter in your pad. No hole in the pad = no hole drilled.

Adding to what markrages already said: most board fabs will plate any copper that is over or directly touches the edge of a hole or routed feature. So a 'standard' way of defining this is just to have copper over the edges of your castellations. This is how vias and plated through holes work - and conversely, this is how non-plated holes work as well (you just pull back the copper a little bit from the hole).

This will work fine with very cheap board houses that are hard to communicate with (e.g. iteadstudio, pcbcart, makepcb). Almost always at no extra charge, it's just part of the workflow. However, if you are doing a production run you will definitely want to communicate any and all special features. Make sure that it is abundantly clear what you mean, use pictures and highlight what you want in the pictures as well as in the gerber files. That will save you from having to do multiple test runs (and waste everybody's time) before you go into actual production.