I've got a set of these: Ay Up Lighting Systems

To answer your selection criteria:

- Bright enough to ride on very dark suburban roads.
✔ High beam with wide lightset will easily illuminate the width of a suburban street and the road for about 5m. Signage illuminated for a good 20m. They also have intermediate and narrow beams which have longer reach. Intermediate is like a search light in my experience.

- Decent battery life - at least 2 hours on high or medium.
✔ 3 hours with half-size battery; 6 with full-size

- I'm ok with a separate battery with a strap or not
Separate battery, attaches with Velcro straps.

- USB charge would be nice if max battery life is less than 2 hours on high so I can charge at work
✘ Charger runs off 12V. It comes with a 110-240V AC mains power supply, there's a 12V cigarette/merit socket cable option for charging from a vehicle. It's a pretty standard DC barrel connector, so you'd be able to get a second power supply easy enough.

I've charged mine successfully off the 12V supply on the bike as well as from solar when on the move.

Someone good with electronics could probably make a 5V→12V step-up converter that would let you charge it off USB but I think it'd be pushing the limits of USB.

- Helmet mount and bar mount
✔ Both available, also a headband mount so you can use it as a regular headlight.

- At least 700 lumens
✔ "over 700 lumen" claimed. I haven't measured this, but my experience has been.

- Also has a blink mode
✔ When turned off; you hold the button down and it'll start blinking or just press it momentarily and it'll turn on solid.

- Reasonably waterproof
✔ These can apparently be submerged to a metre or so, haven't tested this.

A set I'd recommend steering clear of is this one: A headlight to avoid » Blogospheric Refraction