I have been desperately searching for a Joey Safe harness but looks like they've stopped distributing it-- their website has been down for months now. While I know it's shocking for most, it's the next best thing to allowing a child to sit with a lap or lap/shoulder belt alone or even in your lap.
I can't say I'd be comfortable using it for my own kid but... 20-50 pounds http://www.joey-safe.com/cart/freedomride-joeysafe-child-safety-harness.html
It says it can be used with just a lap belt or a lap/shoulder belt.
:shrug: still a better choice than riding unrestrained I think.