Well, I decided to jump in and figure it out.
The answer is this: you take the entire carriage apart...and that includes removing the roll pins that retain the drive gears. Once you're in that far, the coil spring is easy to access...and since it's not under a great deal of tension at rest I don't think that an assembly jig is used to install it. However, if you don't have a vise and some roll pin punches you're going to be in for a hell of a fun time; I had them and it was simple. I even found the retaining nut for the screw buried in some grease inside of the spring.
A word to anyone with one of these enlargers: check the gearbox if you ever have to take the thing apart...mine only contained the desiccated remains of gear oil. I cleaned out the sludge and added a bit of 75w-140 and the worm drive immediately acted a lot happier.
Now I can work on an LED head.