Thanks for the ideas.
Going to try, in order:
Bouncing light off ceiling because it's easiest. If that doesn't work, then:
Find a 7.5w bulb. If that doesn't work, then:
A variant of David A. Goldfarb's idea... I'll try to restrict the light by making an aperture. I know the opening size of the existing cone, I can calc out a circle with half the area and thus half the light. Since we're talking just seconds, and the 15w bulb doesn't generate much heat, it should be OK.