Found something kinda unique while browsing the local ‘trade market’. I guess they are called ‘Lightups’. Either way, they were only $0.20 ea, so I bought them all. They use 2x CR2025 3v lithium cells in series, with a 5mm white (real ugly cool white actually) pcb mounted LED. There is a resistor on the pcb, allowing only 4.3-4.7v to the LED. I am warming up my soldering iron now, plan is to put an XML there instead, think it can be done???
Stay Tuned! J)
Some of the things I’ve seen on this forum I would believe anything is possible with the right person doing it. 
My cousin used those for illuminated centerpieces at her wedding reception. One of those buried under some clear acrylic rocks inside glass vases. Makes for a neat glowy effect. Forget an XM-L. Try a MT-G2 or SST90 with a maget on the bottom and turn them into throwies.
LOL, not sure how practical this is, definitely not near as efficient, I noticed dimming after 1-2 minuted. Oh well, if freaking works, that’s all that matters!! 