SNMP Extend not working

I've got a server running Postgresql 9.1 replication. I wrote a script which prints out the current replication lag (master inserts a unix timestamp every 60s, and the slave compares it to the current timestamp).

I've added

extend replag /usr/local/bin/check_lag_quietly.sh

to snmpd.conf, and restarted the snmpd service, but when I snmpwalk the host, either from localhost, or from another machine on the network, I can see all of the other OIDs, but not this one.
If I do SNMP-Get as

tom.oconnor@charcoal-black:~$ snmpget -v2c -cpublic dns-2 UCD-SNMP-MIB::extTable
UCD-SNMP-MIB::extTable = No Such Object available on this agent at this OID

Then it can't find the OID there either.

Net-SNMP version 5.4.2.1, Ubuntu 10.04.

On Debian with 5.4.3 snmpd dosn't seem to load all it's modules by default, by adding:

-I interface,ifTable,ifXTable,cpu,cpu_linux,hw_mem,extend,versioninfo,snmp_mib,ip,at,system_mib

to the command line for snmpd (note the 'extend' in there), i can:

snmpwalk -Ou  -v 1 -c public localhost nsExtendOutput2Table

which returns the default 'extend' examples:

enterprises.netSnmp.netSnmpObjects.nsExtensions.nsExtendObjects.nsExtendOutput2Table.nsExtendOutput2Entry.nsExtendOutLine."test1".1 = STRING: Hello, world!
enterprises.netSnmp.netSnmpObjects.nsExtensions.nsExtendObjects.nsExtendOutput2Table.nsExtendOutput2Entry.nsExtendOutLine."test2".1 = STRING: Hello, world!
enterprises.netSnmp.netSnmpObjects.nsExtensions.nsExtendObjects.nsExtendOutput2Table.nsExtendOutput2Entry.nsExtendOutLine."test2".2 = STRING: Hi there

Which is close to what you want.

You can get a list of all the snmpd modules with:

snmpd -Dmib_init -H

I guess the real question here is "How do i get snmpd to load all it's modules by default"