The latest beta has IRCv3 nested batch support.

As most servers do not support this feature, /parseline can be used to simulate it in mIRC while connected to a server.

Code:
// simulate server support of these tokens
t {
  parseline -itqp :irc.host CAP * ACK :batch
  parseline -itqp :irc.host CAP * ACK :draft/labeled-response
}

// simple batch events mixed with non-batch events
t10 {
  parseline -itqp :irc.host BATCH +simple
  parseline -itqp @batch=simple :nick!user@host NOTICE $me :Message 4
  parseline -itqp :nick!user@host NOTICE $me :Message 1
  parseline -itqp @batch=simple :nick!user@host NOTICE $me :Message 5
  parseline -itqp :nick!user@host NOTICE $me :Message 2
  parseline -itqp @batch=simple :nick!user@host NOTICE $me :Message 6
  parseline -itqp :nick!user@host NOTICE $me :Message 3
}
t11 {
  parseline -itqp :irc.host BATCH -simple
}

// nested batches
t20 {
  parseline -itqp :irc.host BATCH +outer
  parseline -itqp @batch=outer :irc.host BATCH +inner
  parseline -itqp @batch=outer :nick!user@host NOTICE $me :Message 2
  parseline -itqp @batch=inner :nick!user@host NOTICE $me :Message 1
}
t21 {
  parseline -itqp @batch=outer :irc.host BATCH -inner
}
t22 {
  parseline -itqp :irc.host BATCH -outer
}

// interleaving batches
t30 {
  parseline -itqp :irc.host BATCH +1
  parseline -itqp @batch=1 :nick!user@host NOTICE $me :Message 1
  parseline -itqp :irc.host BATCH +2
  parseline -itqp @batch=2 :nick!user@host NOTICE $me :Message 4
  parseline -itqp @batch=1 :nick!user@host NOTICE $me :Message 2
  parseline -itqp @batch=2 :nick!user@host NOTICE $me :Message 5
  parseline -itqp @batch=1 :nick!user@host NOTICE $me :Message 3
  parseline -itqp @batch=2 :nick!user@host NOTICE $me :Message 6
}
t31 {
  parseline -itqp :irc.host BATCH -2
}
t32 {
  parseline -itqp :irc.host BATCH -1
}