""Would the McE2S fit in place of the McClicky? And would it work with the stock Arc board?""
I have been pondering about this very same question.
First, the part I can answer.
The first run LS board is a voltage boost board. It will get dimmer with a resistor in the circuit. I have not played around enough to dial in a value yet for an optimal 2 stage, but it will work.
The first run hybrid and all the second run (LS2, LSH, LSL, LSHP, etc.) use a Badboy circut, and it will also dim nicely with a resistor in series. Anywhere between 5 and 22 ohms should work, depending on how dim you want the first stage to be.
Would it fit? That will depend on 2 things. How the z-axis space allotted for the McClicky threaded area (about 6mm needed) compares to the z-axis space required by the McE2S (about 6.5mm needed) and how big the diameter of the hole in the aluminium ring that provides continuity for the McClicky is. I suspect the problem, if any, will lie here. The battery end of the McClicky will fit in a 12mm hole, the McE2S requires a 13mm hole.
Yaesumofo will have to answer that part for you.