I am working on a library that tries to make that indexed access easier. It uses mostly JPA standard stuff, but currently I am only testing against hibernate.
Maybe the library can help you. https://github.com/Blazebit/blaze-persistence
What it does under the hood is, it converts accesses like map[key] to something like LEFT JOIN map m WITH KEY(m) = key. Maybe you can use that as workaround if you don't want to depend on another library.