I'm assuming that it's to be driven to full power (@3amps). In that case, the ideal design would entail a copper tube just barely larger than the 18650 cell, with a solid connection between the head, heatsink, and body. The idea here is to use all of the torch to help dissipate heat, so threaded connections would be a hinderance. Like you've planned, using a qtc would definately help to save space, along with a very small reflector or TIR lens to help keep OAL at a minimum. Using the coin, if it's pure copper, seems like a good idea, but I think you may want something thicker with better contact with the body (I'm not sure how snug the fit is now). Interesting mod so far, keep us posted!
I wonder if someone had the idea of making a 1AA powered XM-L flashlight. Since eneloops can be stressed to around 8A draw a good booster circuit could easily deliver around 2A to the led. Don't see a point having a flashlight with runtimes like 5min but heck many things were done just for the fun factor. :)
Before someone try that i'm unaware of any drivers suitable for that operation. I also think it's the cheesiest idea of all. But might be fun. I bet Match does not mind doing one just for the fun factor. Making it QTC enabled might be worthwhile.
I see many problems to make a QTC switch reliable in non twistie flashlights. Just switching battery might be a disaster (different battery length). I'm sure somone tought somethng to counter the effect... yes/no/Foy?
I finally got XM-L from lck-led so I finished modding TR-801. Works great, doesnt screew all the way, but that will be fixed in the future (with some switch modifications). I know my soldering skills are not so good, but hey it works and it works great - QTC even with spring in the tail is great for adjusting brightness (stiff spring), I also tried direct drive and it pulls just a little under 3A@4,1V. In the end I used alumunium coin - 20 haliers (old slovakian currency - 0,2 koruna) on which I mounted XM-L.
Pics (yea I really need to get some new reflectors):
You are right Budgeteer - we need to invent more reliable way to use QTC inside the flashlight... I am using foam rings around QTC, so far so good, but I know that isnt really reliable. Maybe sendwich betwen two plates would be better...
EDIT: I tested it for 15 minutes at about 2/3 brightness and the whole body does get quite hot - around 40-45 degrees C. So heat transfer seems ok.