Как насчет следования?
<source>
@type dummy
tag dummy
@label @INPUT
</source>
<label @INPUT>
<filter>
@type record_transformer
<record>
username user1
</record>
</filter>
<match>
@type rewrite_tag_filter
<rule>
key username
pattern /^(.*)/
tag $1.${tag}
</rule>
@label @OUTPUT
</match>
</label>
<label @OUTPUT>
<match>
@type forward
<security>
self_hostname output.testing.local
shared_key secure_communication_is_awesome
</security>
<server>
host 127.0.0.1
port 24224
username user1
password yes_this_is_user1
</server>
</match>
</label>