This patch can be applied to the mysql client in Percona-Server-5.5.28-rel29.3 to add a new option "--identifiable-comment" (defaulted to on) that appends a comment in the form of "user=os_username&host=source_host&pid=pid_of_mysql_client" to every query sent from the mysql client.

mysql_client_identifiable_comment.patch

--- client/mysql.cc  2013-01-06 22:29:49.000000000 -0800
+++ client/mysql.cc	2013-01-10 20:31:12.000000000 -0800
@@ -42,6 +42,7 @@
 #include <violite.h>
 #ifndef __WIN__
 #include "syslog.h"
+#include <pwd.h>
 #endif
 
 #define MAX_SYSLOG_MESSAGE 900
@@ -140,8 +141,8 @@
 static MYSQL mysql;			/* The connection */
 static my_bool ignore_errors=0,wait_flag=0,quick=0,
                connected=0,opt_raw_data=0,unbuffered=0,output_tables=0,
-	       opt_rehash=1,skip_updates=0,safe_updates=0,one_database=0,
-	       opt_compress=0, using_opt_local_infile=0,
+	       opt_rehash=1,skip_updates=0,safe_updates=0,identifiable_comment=1,
+           one_database=0,opt_compress=0, using_opt_local_infile=0,
 	       vertical=0, line_numbers=1, column_names=1,opt_html=0,
                opt_xml=0,opt_nopager=1, opt_outfile=0, named_cmds= 0,
 	       tty_password= 0, opt_nobeep=0, opt_reconnect=1,
@@ -1548,6 +1549,11 @@
   {"i-am-a-dummy", 'U', "Synonym for option --safe-updates, -U.",
    &safe_updates, &safe_updates, 0, GET_BOOL, NO_ARG, 0, 0,
    0, 0, 0, 0},
+#ifndef __WIN__
+  {"identifiable-comment", 'O', "Append an identifiable comment to queries",
+   &identifiable_comment, &identifiable_comment, 0, GET_BOOL, NO_ARG, 1, 0,
+   0, 0, 0, 0},
+#endif
   {"verbose", 'v', "Write more. (-v -v -v gives the table output format).", 0,
    0, 0, GET_NO_ARG, NO_ARG, 0, 0, 0, 0, 0, 0},
   {"version", 'V', "Output version information and exit.", 0, 0, 0,
@@ -3005,6 +3011,52 @@
   return 0;
 }
 
+#ifndef __WIN__
+/*
+  Creates an "identifiable comment" to append to a query.  This comment will be in the form of:
+    user=os_username&host=source_host&pid=pid_of_mysql_client
+*/
+static String build_identifiable_comment()
+{
+    String comment;
+    const char *prefix = " /** ";
+    const char *suffix = " */";
+
+    comment.append(prefix);
+    comment.append("user=");
+    struct passwd *pw;
+    pw = getpwuid(geteuid());
+    if (pw)
+    {
+        comment.append(pw->pw_name);
+    }
+    else
+    {
+        comment.append("");
+    }
+    
+    char hostname[1024];
+    hostname[1023] = '\0';
+    int gethostname_success = gethostname(hostname, 1023);
+    if(gethostname_success == 0)
+    {
+        comment.append("&host=");
+        comment.append(hostname);
+    }
+    
+    unsigned int pid_length = sizeof(pid_t);
+    char pid_str[pid_length];
+
+    sprintf(pid_str, "%d", (int)getpid());
+    comment.append("&pid=");
+    comment.append(pid_str);
+
+    comment.append(suffix);
+    
+    return comment;
+}
+#endif
+
 /*
   Execute command
   Returns: 0  if ok
@@ -3058,6 +3110,14 @@
     return 0;
   }
 
+#ifndef __WIN__
+  if (identifiable_comment)
+  {
+    /* Append an identifiable comment to the end of the query */
+    buffer->append(build_identifiable_comment());
+  }
+#endif
+
   timer=start_timer();
   executing_query= 1;
   error= mysql_real_query_for_lazy(buffer->ptr(),buffer->length());