Thanks for all the feedback.

I'm looking at a couple of options. One is something called the Romer Vario (also known as the Britax Laptop), which is an energy-absorbing restraint that sits on top of the child and works very well with a lap belt (or shoulder belt). It's more portable than a car seat and would allow us to put our son in the middle seat, which would be safest, when there's a lap belt there. But they're discontinued in the U.S., and perhaps also in Europe. I see some on sale on German eBay, though.

The other option I've found is the RideSafer Travel Vest. You can see it here. It's similar to the JoeySafe but seems more reliable, with good tests and reviews. But it requires a top tether anchor to be used with a lap belt, so that means we'd have to put the child in the more exposed left or right side seat with the shoulder belt. Still, better than nothing, and for occasional use, probably acceptable.