Strong +1 for this idea! I haven't reviewed the patch yet.
When I started looking at NIO2 conversion for Solr (in general, not limited to this class), I noticed a lot of code that concatenates strings of harcoded filesystem or resource paths with forward slashes. The code would be much cleaner and cross-platform with resolve and other NIO2 methods.
I personally would be OK with simply changing the API, but the javadoc at the class level does not actually say that it is expert or internal. I'm guessing deprecation will be required, unless it's sufficient to add the internal/expert designation in the javadoc at the same time as this change. I do see one method currently marked as expert, but I don't think that method is affected by this patch.