The MN03 might not like the much cooler operating temperatures provided by a single battery. The bulb may blacken much more quickly than you would expect, because the lamp will never get hot enough for the halogen cycle to initiate so any and all evaporated tungsten ends up condensing on the inner surface of the bulb.
Or, in laymen's terms... it won't work. Putting an incandescent bulb designed for higher voltage into a lower voltage situation will NOT provide a HOLA experience, I'm afriad.
I just tried it for a few seconds. I put my E2 head on my E1 body. It does light but is very dim and very yellow, maybe one quarter the output at best, of the E1 head. Don't think I'll try the E1 head on the E2 body.
I had thought about a higher output E1, and I found it.
The E2.
Actually, I have a couple of the MN-02 bulbs on order for the E2, which would probably be what a HOLA for an E1 would produce anyways.
The only bulbs I've heard of for the E2 was the standard and LOLA. The pyrex bezel brought alot of speculation of a HOLA for the E2, but PK and, I believe, another person or two from SF assured us it wouldn't happen.
The LED bezels, like the MN-02, might be a great go-between for the two lights.