I also like the idea. Sounds useful for those who want to further customize their timestamp.

I made a handy little snippet that uses a timer on START to find the exact time each second starts in $ticks. Some basic subtraction, and voila! You can have milliseconds in your timestamp

on *:START: {
  unset %t
  .timer $asctime($calc($ctime +60),HH:nn) 1 0 msget 1 $!+ $!right($ticks,3)
alias -l msf return $right(0000 $+ $left($1,$2),$2)
alias msget {
  if (!$isid) set %t $1
  else {
    var %c 1 $+ $right($ticks,3)
    var %c $calc(%c -%t)
    if (%c < 0) { var %c $calc(1000- $abs(%c)) }
    return $replacecs($asctime($1),xxx,$msf(%c,3),xx,$msf(%c,2),x,$msf(%c,1))

It probably has an accuracy deficiency of +-10ms but in timestamps, that's not much.

If we could use identifiers in timestamps, you'd be able to do /timestamp -f [HH:nn:ss $msget(xxx) $+ ]