I think all (or most) of the REST/HTTP libraries are in Python so far, but it shouldn't be too hard to create a Java version if someone puts the time into it.
As for REST Assured, if I'm not mistaken, that is like a Java library that you can import to use to do REST testing within Java code, so omitting the testability aspects of assertions within that tool/library, etc. you can wrap the (generic) library functionality into a RF compatible Java (wrapper) library that you can then use with RF over Jython or the Java remote (library) server. Though that would mean you may not easily reuse existing tests as is, it would be no different than using the other RF REST/HTTP libraries, with an exception that you can reuse your existing test code a bit if you customize the wrapper library to be specific to your REST APIs that you're testing (which would call parts of your existing test code adapted for RF for resuability).