Having struggled to get water tight seals on both my Nordic stock pot & bain marie, I got a friend of a friend to knock me up a 55mm long, custom brass 1/2" BSP thread runner/nipple, which had 25mm of thread one end, followed by a 5mm unthreaded gap, followed by another 25mm of thread at the other end.
With hindsight, I should have got him to leave a slightly bigger gap i.e 6mm to 8mm.
I used these measurement, as my idea was to keep this as perminant fixing, allowing me to add/remove the ball valve (likewise with the internal hop strainer). As such, it needed to be long enough to accomadate two locking nuts. I planned to use silicon O rings, as I've used these to good effect on my thermometer on my existing boiler.
Then on one end, it was 'rebated' to take a standard 15mm olive, for use on a compression joint. The whole thing had an internal diameter of 15mm.
Since there was still a chance of liquid ingress into the inner thread, I'd already planned to solder the inner lock nut in place. This worked better than I expected, as I got an excellent flow of solder, right the way through the joint.
The length gives a full thread into my ball valve + leaves enough 'slack' for the two lock nuts & associated O rings.
On final fitting, I decided to only use one O ring on the inside, as using one on the inside & outside, gave a little too much 'flex' when operating the ball valve. So instead of the outside O ring, I used a fibre washer, I then used a second fibre washer between the outside lock nut & ball valve, as this not only protected against leaks but also made it easier to remove the ball valve.
Since the change to the outside fibre washer, there is minimum flexing of the joint & not matter how hard I twist, pull, flex, turn the joint, it remains water tight, even with a 50L 'payload' in it
As this was custom made, I should be able to get any length/gap etc combination made up. This item wasn't that cheap, as the bloke making it, had to buy in the materials, die & 15mm drill bit but since he now has them, he's advised that any requests will be significantly cheaper.
So I'll be ordering again (with a larger 'gap' area), in the non too distant future, if anyone else is interested?
Happy brewing
Darren
& even if there is an alternative, it won't P*$$ on my parade, as half the enjoyment of my beer brewing, is this kind of thing & the very fact there's a similar offering, just goes to prove my idea/thoughts, was right on the money 
Your idea is very similar to a weldless spigot, I use them on my boiler for thermometer, sight tube and valve.
of quid