Hi Colin,
It's been a while since I used them, but I think POST /Info/Filters returns some information on dimensions/hierarchies/levels, and POST /Info/FilterMembers returns information on the members of a level. If you need metadata on properties specifically, I'm not sure if that is included. I don't think we have information on namespaces as part of the REST API for IRIS BI, but maybe someone has a suggestion of other ways to get that kind of information about an IRIS instance over REST. Anyway, I'd recommend taking a look at those two /Info services, and possibly opening a WRC issue if you find that they don't provide what you need.