Seems like a purely legal problem to have some form of real world handle. For such problems, I'd also expect a legal solution.

For example, the license could include something like: if we cannot identify and contact you in a reasonable amount of time, you have no say. Or: contributions from a certain amount of lines of code require a stable contact etc.

I don't see a problem of liability in general. And for special cases, one can still require verification. I can very well imagine that people develop critical software open source. For example, a governmental app, or a software for a nuclear power plant. Then, the public can also have a look and see how things are operating. However, open source doesn't mean that anybody is invited to contribute IMHO.