If someone can come up with a solid, deterministic rule for how this can be done, I can implement it.
Does it need more than an index operator to allow users to specify which collection element they refer to? I mean, the POM is XML, XML is a sequence of elements so all collections should in fact be lists that can be accessed via a zero-based index.
The brackets that Java uses for array indexing are forbidden in URI paths but there are still other available delimiter characters. So, how about something like one of these: