I am in a catch-22, it seems. I followed instructions here,

  http://www.quickfixj.org/quickfixj/usermanual/1.5.0/usage/jmx.html

in order to enable remote jconsole / JMX controls on my FIX session.
However, because there is a seqnum mismatch, no session appears in jconsole,
so I cannot set the sender-seqnum!

In summary: for remote jconsole / JMX controls to resolve a seqnum mismatch,
it would be helpful if the NextSenderMsgSeqNum could be set independently
of whether there is a current mismatch. Otherwise, the jconsole / JMX controls
are useless in this scenario.