A diode in series would be a half wave rectifier, - not suitable, with no effect on inrush and an rms voltage of 78 volt ( on a 110 V circuit).
I would say that while it seems simple to add a series resistor ( or inductor) , the calculations must be done properly or it won't do the job, and it must be rated and istalled safely to avoid shock or fire hazard. Not so simple to implement. Resistor will run hot and generally, a resistor should not be used at more than about half its rated value, to reduce high temperatures on its leads and surface.
My vintage Federal condensor just has a standard lamp holder. It had a now obsolete Sylvania 40 Watt 110 V lamp which failed , so have been using a standard 75 Watt lamp with the lettering rubbed off.
I modified the bellows to take M39, and I use a 90mm for 6 x 9 and a 50 mm for 35mm. Using Ilford Multigrade RC Pearl 8 x 10 (inch), the results are good so far.
I have here some "Hitlights Afford 1X LED light bulb" " rated 9 Watt with color temp 6000 K. Next time I use the enlarger I might try one, to see if it gives even light across the frame ( I use a dslr to check) and still have the nice tonality on the prints.
-Anybody using household LEDS ??
The problem with the original lamps in the old enlarger is that the negs heat up while I fool around focussing etc.
So LED might help that too.
CFLs are not so good because they take a few seconds to even out, also they can have residual glow when off.