Pretty straightforward:

$ (has-param "foo")
nil
$ (set-param "foo" 42)
42
$ (has-param "foo")
t
$ (get-param "foo") 
42
$ (delete-param "foo")
$ (has-param "foo") 
nil

The S-XML-RPC library is used for communicating with the parameter server and converting XML-RPC types to Lisp objects. Most of the type mappings are straightforward: strings to strings, arrays to lists, etc. See the table at the link above for the full list.