im nt too sure if any script could stop you from pinging out,
if your client is frozen, you wont be able to reply to pings so youll end up being disconnected.
mIRC also has its own ping pong thing im pretty sure
try this and let me know how it goes, i cant test it too much as i dont have a idle or ping problem

menu * {
  Ping-Pong:$iif(%pp,antioff,pingpongon)
  Anti-Idle:$iif(%antiidle,antioff,antion)
}
on 1:ping: { $iif(%pp,raw pong $1 wannaplaypingpong,) }
on 1:pong: { $iif(%pp,raw ping $1 wannaplaypingpong,) }
alias pingpongoff { unset %pp | echo -a Ping-Pong has been disabled. }
alias pingpongon { set %pp on | echo -a Ping-Pong has been enabled. }
alias antioff { timeridle off | unset %antiidle | echo -a Anti-Idle has been disabled. }
alias antion { timeridle 0 120 scid -atM1 antiidle | set %antiidle on | echo -a Anti-Idle has been enabled. }
alias antiidle { msg $me Anti-Idle }