It appears from some limited testing that the original caller id is always

shown when the call is transfered.  Is there some way to have the person

making the transfer show up as the caller id?

To answer my own question it appears that the information is available
in the sip_h_Referred-By variable.  E.g.:

 <extension name="system25_park">
    <condition field="destination_number" expression="^\*5$" />
    <condition field="${sip_h_Referred-By}" expression="^<sip:([0-9]{4})@.*$">

allows the station id making the transfer to be known when a call is
transfered to *5.  The station id can then be used to park the call in
the proper fifo.