I usually radio trigger them anyway but a hot one could fry your radio receiver at the flash end.
On another forum, someone suggested this was the case with the cheap Cactus radio triggers. I took mine apart and looked up the data sheet for the opto triac they use and found that it was rated at 400v.
I can dig up my notes if you are at all handy with DIY electronics fiddling.
You place a potentiomenter across two of the front plug in terminals, and then you can dial it back to anything as little as about 1/12 power repeatably.
I just soldered stout wires onto the wiper and one track lead of the pot, and bent them to suit, and plugged them in.
It stayed on the front of the unit from the friction fit of the two wires in their sockets.
I added and old knob with pointer to the pot shaft, and fashioned a dial face.
I marked calibrations on the dial face after firing the thing into a flash meter a number of times to figure out what the varied settings on the pot were giving me, light output wise relative to full manual output.
All up about 1 hour of effort, and maybe $3 at the electronics wholesaler.
I am beyond handy, IPC-610c certified and rework IPC-7721 certified. I'm game.
Without benefit of notes - here it is from memory-
I worked it out with a VOM, and from studying the continuity of the connections at the remote sensor cord, with the sensor plugged in and also removed.
Two (of the four I am recalling) of the contacts are the send and receive for allowing a hot shoe or PC socket connection made at the far end of the remote sensor cord to trigger the flash.
One of them shows up as a short circuit using the ohm meter with the flash turned off, between it and the flashes' shoe return contact.
The other of the four connection terminals for the sensor turns up at the same potential between center post contact on the hot shoe to edge connection.
On the auto sensor the pair for flash send and recieve are shorted - zero ohms. You can confirm this by trying to fire the flash with the sensor pulled - it won't.
The remaining two terminals on the remote sensor place different resistances across the terminals to vary the light output.
The resistance between the normal sensor pair of terminals will be quite high when the sesnor eye is taped over and then will fall when pointed at a bright light source.
Changing the ND filter by rotating the barrel changes the amount of light that gets at the sensor, hence the amount of resistance it presents to the flash duration circuitry inside the flash.
So finding the resistance terminal pair should not be too hard.
I think I used a 250K linear pot; there was a useless portion on the rotoation where the resisitance was too high. You may want to try a potentiometer with a lower resistance; I don't know how much less though.