There is a 'Royal Blue' LED with a wavelength of 455nm. It should be a bit better match to paper sensitivity, but I don't know how much effect it will really have. My LED head is not at that point yet.
LED light output in lumens is basically proportional to the current you put through it. A potentiometer/rheostat will work, but will dissipate a good deal of heat and doesn't really regulate the current.
I have a simple (to me, at least), low cost (< $15) linear drive circuit for these LED's. My end goal is have a LED head for a Durst 138. I'm starting with 4 LEDs( 2 green, 2 royal blue) to figure out the best approach - whether to directly replace the bulb, much like the original poster, keeping the condensers, or make a mixing chamber as in a color head. Huw's approach just seems like too much work to me.
I'll happily share the schematic if anyone wants to experiment with the LEDs. If you can solder, you can build this pretty easily.