http-binding and ExternalIdentifier / RIM targets

Clarification sought.

I just went to use the http-binding GET method with the JAXR and it 
appears to be hardwired to only work with UUID instead of
the more logical ability to query against any column in the RIM, and 
specifically of course the ExternalIdentifier - which one would
expect to be the most logical place to query from a GET - because 9 
times out of 10 you could not expect to know what the
UUID is even care necessarily (it could change for updated items).

Anyway - is this a behaviour of the JAXR implmentation - or did the 
http-binding GET specification only allow UUID as a target?

What have other people implemented in their http-binding interfaces?

If this si a minimalist detail in the specification - we probably need 
to fix this in the specification to allow any RIM target column. 
(it is not clear from the current documentation what "id"  is that is 
referenced).

Thanks, DW