Can a small common box ball stay permanently in a 12" diameter pot?

In short no. You will have to use some of the techniques that Bonsai growers use to keep it happy/healthy. Root pruning is prime in this along with pruning the top. You don't of course have to create a little miniaturised tree but keeping it trimmed and root pruned (every second winter) is key. Feed regularly too.

I would look at root pruning on a Bonsai site to see the details. If you do this, then yes, you can keep it in the same pot indefinitely.

Thank you for that Fractal. So it is workable. I also have a dwarf edging box (I forget the correct name for it) in a much smaller pot. It has been there for several years and the box is about 8" x 8" domed. Looks attractive and healthy. I've had it out of the pot recently to inspect the roots and there is a little bit of congestion. Should I also root trim this a bit come the winter?

Yes, gently tease out some roots and lightly snip away the longest bits. Always replace the soil with new, preferably a loam/compost mix for Buxus. Bonsai growers use a pointed stick about the thickness of a large knitting needle (if you are familiar with one of those Lol).

Hi Jonathanhales,you should be ok following Fractals advice (nice one, Fractal).
The pointed stick (knitting needle?, chopstick, bbq skewer, pencil, etc.) is used to poke the growing medium down to the roots so as to leave no air pockets.
This root pruning technique can also be used for most of your shrubs in pots.
Fortuitous expectations to you.

Ah, most ingenious wigglings of chopsticks. Another method would be to turn a rootball upside down. Then make a wet mudcake on top of it, and replace the pot onto it from above. Try doing whilst standing on head.

Fell over many times... with practice I dunnit. I find placing one foot behind each ear helps with your balance though. ;-))