Nice work!
I'm thinking you might seriously shorten an LED's life running 12v through it. I'm no expert, but it seems putting more than maybe 5 volts across the LED is counter-productive. Perhaps someone with real expertise can answer that point here for us.
There's a relatively easy way to put a resistor in-line. Just use a short piece of wooden dowel cut to fit inside the case between the battery and one contact.
Trench out one side of the dowel to hold the resistor. Push a thumbtack in one end, pull it back out, stick one shortened lead from the resistor in the hole and put the thumbtack back in. Cut a finishing nail pretty short and use the head on the other end ... do the same thing with the other resistor lead. Glue the resistor in place. I wrapped the dowel in a single sheet of paper to give it a slightly more snug fit in the barrel.
Granted, this is more effort than putting in a spring, but the current limiting resistor will keep the LED from degrading.
I have three issues with the single LED Solitare conversion to overcome:
1) The LED deforms the plastic lens slightly when turning off the light. Not a big deal, but I can't guess the effect over time.
2) As a direct effect from the pressure applied to the LED, after MANY on/off repitions, the torque caused one of the LED leads to break off right at the resin. I now only use CHEAP LED's in the Solitare.
3) Its a fun project, but the Photon or its competition makes a lot more sense for me in real world application.
Of course, if you share the same obsession as many of us who visit this site, 'real world application' doesn't have much bearing on whether or not to do the conversion!
When I see a new, conventional flashlight, I immediately start planning how to convert it and most often end up doing it!
What I wish I could create is a flashlight with the Solitare's form factor containing 3 5mm LED's in the lens area (wired in series for 4 volts each) running off a 12v battery & no resistor, while keeping the factory style twist-switch functional. I'm not sure I'm ready to do a custom mold & resin pour to form the part (or that I have the skill in the first place) ... but it would be a very sweet flashlight!