You can deal with this by changing your New Ticket HTML.

Setup->Tickets->New Ticket Page

The text edit has a copy of the html+th1 code for doing new tickets,
at the very top is

<nowiki><pre>
<th1>
  if {[info exists submit]} {
     set status Open
     submit_ticket
  }
</th1>


If you change that to

<nowiki><pre>
<th1>
  if {![info exists username]} {set username $login}
  set pstr "[htmlize $login]"
  if {$username ne $login} {
      set pstr "$pstr claiming to be [htmlize $username]"
  }
  set pstr "$pstr posted on [date]"
  if {[info exists submit]} {
     set status Open
     set comment "$pstr<br><br>$comment"
     submit_ticket
  }
</th1>


You'll get 'so & so posted on somewhen' at the very top of the
first comment in a ticket.

Caveat: I haven't tested this quite as extensively as I might...

Th1 isn't documented much&#8212;yet&#8212;but it's basically specialized Tcl.  You can do quite a bit of customization with it, including changing sqlite tables to suit you.