I ended up coming across a little light that I think fits the bill nicely for neck lanyard use - the Princeton Tec Impulse.

Here it is shown on a lanyard along side a Fox 40 Sharx whistle:



It uses 2 coin batteries, is waterproof (IPX4), and has 3 modes: High - 12 hours, 12 meters; Low - 36 hours, 6 meters; and Flash - 48 hours. Just to note, this mode information is different than found on either the MEC page I linked to or the Princeton Tec page; the specs have changed but the websites still reflect the previous model information.

Also, the clip shown above can be removed and exchanges with a hat clip if desired.

I've used it on a couple trips so far and am pleased with it, especially given the $10 price tag.