As a starting point I would be thinking about looking for a macro that can examine the contents of the bags that you are carrying.
Unless you do something clever this type of macro is going to out grow you 255 character limit very easily.
Well, not necessarily. It's fairly trivial to loop through the contents of your bags in a macro. I don't think it would take a lot of characters to check the type of an item within a loop, and cast milling. I do, however, believe that every individual milling will require a confirmatory hardware event. I'll see if I can think something up this afternoon after my exam.
It looks like Apaseall is correct. This isn't possible using a macro due to the character limit. I *think* the AutoDE feature in Enchantrix will also Auto-mill, so you could use the macro posted in this thread to quickly turn that feature on and off when you're ready to do your milling.