Digitally measure current on very high voltage line
You can buy a ferrite toroid core such as T58/41/18-3C90 58mm OD that should leave plenty of room for primary and secondary windings to provide power. Not many turns for a few volts at each end. LT3439 could be used if you like pricey chips. Or find an old TV flyback transformer core. Be sure the insulation is adequate for the voltage and the windings can't get out of place, and ground the low voltage side carefully. 

For HV optical signal isolation you could use a low baud rate ( 1200 or 2400 baud should be fine) and IR communication using a light pipe or free space. It would be especially easy to use the 38kHz IR remote frequency. 



However! The meter is usually on the return side of the laser tube which is near earth potential, in which case you don't need to provide much in the way of isolation. You can bypass the shunt with a TVS or similar if you're worried about it getting disconnected.