The L1D, L2D, and P2D all use the same head. While you can use a 14500(4.2v fully charged) you will lose the lower levels(you get SOS, strobe, and all brightness will will be the same-Turbo until the battery voltage drops to ~3.2v). The head uses a boost circuit and above ~3.2v the light goes in direct drive.
Sounds definitely interesting. What's the best brand name in a protected 14500 battery? What charger would you recommend?
I've seen a number of people recommending the Pila charger but their practice of renaming battery sizes leads me to believe they are not a company I would want to do business with.....
AW's batteries are very good. You can buy them direct from AW here http://www.cpfmarketplace.com/mp/showthread.php?t=173892 or you can get them from Lighthound. I hear(read) good things about the Pila charger but haven't tried one yet. I am currently using the Ultafire WF-139(same one AW and Lighthound sells).
I think the other thread was referring to a 3v primary. These can be obtained by disassembling certain Lithium camera batteries. Its not worth it though when you can run the CR123 body and a much cheaper primary CR123.
You can use a LiFePO4 14500 and retain all modes. The only question is, why? The only mode that gets any brighter is turbo, all the others don't change.