I don't believe this is a solved problem, and it's been around since OpenTracing days[0]. I do not think that the Span links, as they are currently defined, would be the best place to do this, but maybe Span links are extended to support this in the future. Right now Span links are mostly used to correlate spans causally _across different traces_ whereas as you point out there are cases where you want correlation _within a trace_.
[0]: https://github.com/opentracing/specification/issues/142