i had this same issue but i have the 504G phone. i was able to use the below custom softkey.
fnc=sd;ext=700@$PROXY;vid=1;nme=Park
then i changed my Connected key to the below.
confLx|1;flash|1;xferLx|2;conf|1;xfer|2;toggle|4;redial|5;dir|6;bxfer|4;park|8;phold;psk2|3
since then we have moved to using FreePBX which now the above does not work correctly. it still parks the call but it announces the park location to the parkee not the parker.
did you ever find a solution?