This is not currently possible, but I'd love to have it implemented in RSTALanguageSupport
's XML support one day.
Contributions are welcome! If you can parse a schema into some sort of usable model, and would like to contribute to RSTA, I'd be happy to include it in the project. With an in-memory representation of a schema, all we'd need then is a method to return all the nested tags we're in (which would be pretty straightforward; I could add that and any other infrastructure), and this functionality could be added.
DTD's would be even easier to add, since they require unique element names. I just might work on that this weekend, just for fun.
Schemas are probably more relevant these days, but this would put the framework in place, and I personally still work on a lot of XML at work that is backed by a DTD.