An incomplete topic, for now.
Some time ago, I heard of these units, made in Italy by a company who also make spheres.
They have a system to repressurise spheres whereby the sphere's end blanking plug is replaced by a ball-and-spring valve assembly. To regas, a gassing head is screwed onto the ball valve back - not unlike charging an AC unit in a car. This head is connected by a flex hose to a regulator, in turn linked to a Nitrogen bottle. Charging pressure is controlled by the regulator knob.
When it's done, a sealing plug is screwed in to the valve back, and a green plastic cap clipped over it all.
Advantages of this system are - as I see it - that you don't need to remove the spheres to regas them, and you can let just a little gas out if you want to experiment with pressures.
Disadvantages are - apparently a slightly higher rate of gas leakage (compared to blank threaded plug regassing), lack of authenticity in appearance, and incompatibility with other recharging systems. Also there is a good amount of nitrogen lost from the flex hose when the gassing head is unscrewed each time.
As supplied, the regassing kit also comes with a nitrogen bottle fitting incompatible with Australian cylinders.
Considering all the negatives, I remind readers I am a stubborn f***er and therefore decided to buy this gear for my own porpoises.
Costs to date:
Regassing rig and twenty ball valves $525 landed
Aussie bottle fitting with mesh insert filter, modified to fit $100
Note there is not yet a line entry for N2, as I will probably scrounge for the first while. You can see, however, that it is not dirt cheap, and for $700-upwards (bottle included) you can buy a lot of club regassings or aftermarket spheres. So, contrary to the usual macho blather of "I just bought this tool and it is AWESOME!!!" this probably doesn't fit your billing unless situation and plans are similar to my own.