I had this same problem. I assume that you have female, half couplings welded to your kettle.
I ended up making my own fitting by brazing a copper tube inside a male nipple that screwed in from the outside. My ball valve then screwed onto this male fitting. Strange looking, but it works.
I think I can see how that would work. Don't you lose a lot of space in the connector? Seems like this would drop 1/2" port down to 3/8" or 1/4" pretty quickly.
The idea about running a tap all the way through the coupling seems like a good one. Assuming there is enough sidewall to support threads both internal and external.
I have a 1" female half coupler welded to my kettle, and since my ball valve is 1/2" ( don't ask) I have a lot of room to spare. A picture of it installed would not tell you anything and sorry, but I'm not disassembling it.
If I ever build another kettle, it will have full couplers in stalled rather than half ones, for this very reason. I'm really surprised that of all the reading I did before I built mine, no one ever addressed this problem. Not to mention that half couplers are much harder to find.