Please apply the following patch to the source tree. It took me some time to figure out the error on the env.<variable> line, and I half expected that munin-node would process the config file each time called, so noting the need to re-start munin-node is useful.
diff -Naur munin-1.3.3/dists/debian/plugins.conf munin-1.3.3-edited/dists/debian/plugins.conf
--- munin-1.3.3/dists/debian/plugins.conf 2006-11-10 12:39:14.000000000 +0000
+++ munin-1.3.3-edited/dists/debian/plugins.conf 2006-11-26 21:57:49.000000000 +0000
@@ -4,9 +4,10 @@
# group <group> # Set the group to run the plugin as.
# command <command> # Run <command> instead of the plugin. %c expands to
# what would normally be run.
-# env.<variable> # Sets <variable> in the plugin's environment, see the
+# env.<variable> <value> # Sets <variable> in the plugin's environment, see the
# individual plugins to find out which variables they
# care about.
+# Re-start munin-node for changes to take effect
[apt]
user root
diff -Naur munin-1.3.3/dists/freebsd/munin-node/files/plugins.conf munin-1.3.3-edited/dists/freebsd/munin-node/files/plugins.conf
--- munin-1.3.3/dists/freebsd/munin-node/files/plugins.conf 2006-11-10 12:39:14.000000000 +0000
+++ munin-1.3.3-edited/dists/freebsd/munin-node/files/plugins.conf 2006-11-26 21:58:39.000000000 +0000
@@ -6,11 +6,11 @@
# group <group> # Set the group to run the plugin as
# command <command> # Run <command> instead of the plugin. %c
# expands to what would normally be run.
-# env.<variable> # Sets <variable> in the plugin's environment, see the
+# env.<variable> <value> # Sets <variable> in the plugin's environment, see the
# individual plugins to find out which variables they
# care about.
#
-#
+# re-start munin-node for changes to take effect.
[mysql*]
#env.mysqlopts -u someuser
diff -Naur munin-1.3.3/dists/tarball/plugins.conf munin-1.3.3-edited/dists/tarball/plugins.conf
--- munin-1.3.3/dists/tarball/plugins.conf 2006-11-10 12:39:14.000000000 +0000
+++ munin-1.3.3-edited/dists/tarball/plugins.conf 2006-11-26 21:57:25.000000000 +0000
@@ -6,11 +6,11 @@
# group <group> # Set the group to run the plugin as
# command <command> # Run <command> instead of the plugin. %c
# expands to what would normally be run.
-# env.<variable> # Sets <variable> in the plugin's environment, see the
+# env.<variable> <value> # Sets <variable> in the plugin's environment, see the
# individual plugins to find out which variables they
# care about.
#
-#
+#Re-start munin-node for changes to take effect
[mysql*]
#env.mysqlopts -u someuser