I've been looking into getting a Kelty Hawkeye Fanny Pack for use in day hikes from a base camp, and to have on the front when carrying my larger pack.

While I haven't tried it out or purchased it yet, it may serve your purpose. Weighs 26oz, but it comes in at 800ci, instead of the measly 400 of the syncpack (at 32oz). Not as much direct support as the syncpack, but you said you would be putting lightweight stuff in it.