Found it! Took a cue from the JavaScript sample and changed the parameter call to read:
iResult=web.reg_save_param("UserSID", _ Array("LB=<USERSID>", _ "RB=</USERSID>", _ "RelFrameId=1", _ "Search=Body", _ "LAST"))
And it now returns:
Notify: Parameter Substitution: parameter "UserSID" = "351345874000113011"