It is already possible in the command line/XML:
List lab controllers:
$ bkr list-labcontrollers
Search for all machines in Boston:
$ bkr list-systems --xml-filter='<hostRequires><hostlabcontroller op="=" value="lab-02.rhts.eng.bos.redhat.com"/></hostRequires>'
Of course, you may use the following snippet in user defined presets too:
<hostlabcontroller op="=" value="lab-02.rhts.eng.bos.redhat.com"/>