some possible scenarios, need further digging:
a) for that user, execute the "record" dialplan application, then transfer to the conference extension. It can be achieved by a1) setting a variable in the user directory entry, and then matching the variable in the dialplan; a2) defining a separate context for the user.
b) an external script would monitor the ESL events and execute uuid_record when needed
From: Matthew Cordes <cordes.matthew@gmail.com>
To: freeswitch-users@lists.freeswitch.org
Sent: Wednesday, August 14, 2013 10:34 PM
Subject: [Freeswitch-users] Recording conference only when a particular user is connected