Turns out that all the Anodes are available and connected up to the anode driver PCB where some of them are left not connected. Same thing with cathodes and cathode driver PCB. So at least my version of MS6205 is open for modifying it into a full 100x100 matrix display.

Bad news is that Sharp and other manufacturers (I'm looking at you Электроника) mostly stopped making arrayed drivers for high voltages, but single or dual transistors are still available, so I'm going to try to leverage that or source some old stock chips.

One more thing, if your neon plasma display begins glowing in pink instead of orange (plus several pixels around it light up in all directions) then the current is too high, check for shorts to the high voltage rail that bypass the pull-up resistor.