It's going to be some months yet before I need a serious light for commuting (ah, those long summer days are upon us!), but I'd like to as least start thinking about it in case a deal pops up somewhere. I have an emergency light which is good for blinkiness and riding very slowly in the dark on the Hudson Greenway (no cars). So I'd like eventually to get one of those big ones that requires a battery and goes in a water bottle cage. Any clever ideas for how to work this or am I going to have to get one of those strap-on water bottle cages for the handle post to hold the battery?

You might wnat to consider a bag-type rather than a bottle-type battery: the battery's in a little cordura bag with a velcro strap or two. You can fit it to the handlebars or stem or wherever.