SST-50 direct drive ??

Can SST-50 be directly driven by an 18650 ?

Yes, but you have to watch out for overcurrent and make sure to use thick wires if the current is to low. : )

Or you might try a Sandwich Driver [ combining 2 drivers ]

I was thinking of going SST 50 , + Sandwich Driver , and then see how much current a single 18650 could supply ...

Some folks are combining 3 or 4 1400mAh drivers with the SST , to get voltage regulation + current , 3 x 1400 would offer 4200mA potential and regulate the voltage ..

As to whether your 18650 could supply such current , that is the question ?

Most cells I have tested are limited to around 3A or worse , the only cells to really push harder = Panasonic 2900 and Samsung 30A [ 4.35v ] ...

So if your not prepared to buy the very best cells available , it may be pointless ...

The XM-L emitter looks to be worthwhile waiting for ...

Supposedly, running two the of DX mc-e drivers (~2.3A each) together works. http://www.dealextreme.com/details.dx/sku.20330

Running direct drive is just ghetto. Even if you get one that works fine on a IMR, it'll only maintain that high current for 5-10 min.