jack wrote:

why not consider a super-capacitor and d.c-d.c converter -- the converter
can loaf along with a very low duty cycle

linear tech had a very low loss dc-dc converter application note which
appeared in EDN in the Spring.

Yep, I'd planned to put an electrolytic near the load; it hadn't crossed
my mind to consider a super cap there. That could be a good match.
I'll look for the appnote.

Now, maybe a dumb question, but... DC-DC converter seems to be a generic
term in this area, with 'boost' and 'step-up' being more specific
synonyms (but I could be reading this all wrong). By DC-DC, do you mean
a linear regulator? Don't they generally have lower efficiency and
higher quiescent current? (This gadget won't have a power switch in
most cases, so the supply itself can't draw too much current when the
LEDs are idle.)

Cheers,
Richard