I would suggest not just an USB isolator, but also a separate safe potential grounding line from the keyboard/USB connector chassis to a grounding point, through a big beefy resistor (100k? 1M? Preferably > 1W rating, so it can handle the high voltage).
The idea is to provide a path to ground for the potential to dissipate through, without disturbing the electronics.
Even better, install a conductive plate, maybe thin aluminium sheet (the nonconductive aluminium oxide layer doesn't matter) for your palms to rest on, or that you always touch before anything else on the machine, that dissipates the static charge to ground (like the computer chassis) through a suitable resistor.
If you have a whiteboard, and in the winter season get zapped by the aluminium shelf you hold the pens in, you can do the same to that too: connect a beefy resistor to the aluminium shelf, and then a single wire from the other end of the resistor to a grounding point. Works very well: touching the shelf will then dissipate any potential difference (static charge) "slowly"/safely through the resistor, and you won't feel a thing.