Basically, a dialog to try and clean up code, removing "untidy-looking" practises such as multiple close brackets on a line, using the pipe for 2 lines on one line, etc.
I made this on a whim, and really haven't done extensive testing. I am sure that there will be bugs, and things I've missed that should be added. Please tell me these! I will follow up on all comments and adjust the script accordingly:

Note: I was indenting the code with tabs, however I found that this caused mIRC to go crazy in the script editor, and indenting with double spaces did not work (it just remained un-indented). If anyone has a solution to this let me know.

/beautify to start, or right click in channel/pm/status windows.

Use at your own risk. Always keep a backup copy, at least until this script is tested more extensively.

alias beautify dialog -m code_table code_table
menu query,channel,status {
  Beautify:beautify
}
dialog code_table {
  title "mIRC Code Tidier"
  size -1 -1 296 262
  option dbu
  edit "", 1, 1 2 295 247, multi return autohs autovs hsbar vsbar
  button "Clean!", 2, 130 249 37 12
}
on *:DIALOG:code_table:sclick:2:{
  var %x 1, %ticks $ticks
  savebuf -o code_table 1 %ticks $+ .txt
  if ($lines(%ticks $+ .txt) > 0) {
    beautify_file %ticks $+ .txt
    did -r code_table 1
    loadbuf -o code_table 1 %ticks $+ .txt
  }
  .remove %ticks $+ .txt
}
alias beautify_file {
  var %f $1,%w @ $+ %f
  window -hn %w
  loadbuf %w %f
  var %l 1,%commented $false
  while (%l <= $line(%w,0)) {
    var %t $regsubex(tab,$line(%w,%l),/^(\s+)(\S.+)$/g,\2)
    if ($regex($line(%w,%l),/^(\s+)(\S)/g)) rline %w %l %t
    if (!$regex($line(%w,%l),/\S/)) dline %w %l
    elseif ($regex($line(%w,%l),/^\/\*/)) || (%commented) {
      var %commented $true
      inc %l
    }
    elseif ($regex($line(%w,%l),/^\73/)) inc %l
    elseif ($regex($line(%w,%l),/^\*\//)) && (%commented) {
      var %commented $false
      inc %l
    }
    elseif ($regex(%t,/^(.+?)\s\174\s(.+?)$/)) {
      rline %w %l $regml(1)
      iline %w $calc(%l + 1) $regml(2)
    }
    elseif ($regex(%t,/^((?:.+?)(?::|\s))\173(\s+?\S(?:.+)?)$/)) {
      rline %w %l $regml(1) $+ $chr(123)
      iline %w $calc(%l + 1) $regml(2)
    }
    elseif ($regex(%t,/^on ([^:]+)\72((?:[A-Z]+)?(?:[a-z]+)(?:[^:]+)?)\72(.+)$/)) rline %w %l on $regml(1) $+ : $+ $upper($regml(2)) $+ : $+ $regml(3)
    elseif ($regex(%t,/^(.+?)\s\175(.+)?$/)) {
      if ($regex(q,$regml(1),/\S/)) {
        rline %w %l $regml(1)
        iline %w $calc(%l + 1) $chr(125) $regml(2)
      }
      else {
        iline %w $calc(%l + 1) $chr(125) $regml(2)
        dline %w %l
      }
    }
    elseif ($regex(%t,/^on (.+)\72\s+?\173/)) rline %w %l on $regml(1) $+ : $+ $chr(123)
    else inc %l
  }
  var %l 1,%tab 0,%com 0,%comment false
  while (%l <= $line(%w,0)) {
    var %temp $false
    if ($regex($line(%w,%l),/^\/\*)) {
      var %comment $true,%temp $true
      rline %w %l $str($chr(9),%tab) $+ $line(%w,%l)
    }
    elseif ($regex($line(%w,%l),/^\*\//)) {
      var %comment $false,%temp $true
      rline %w %l $str($chr(9),%tab) $+ $line(%w,%l)
    }
    elseif ($regex($line(%w,%l),/^\73/)) rline %w %l $str($chr(32),%tab) $+ $line(%w,%l)
    else {
      if ($regex($line(%w,%l),/\175$/)) {
        ;dec %tab
        var %temp $true
        if (%com == 1) && (menu* !iswm $line(%w,$calc(%l - 2))) {
          rline %w $calc(%l - 2) $remove($regsubex($line(%w,$calc(%l - 2)),/^(.+)\173$/,\1 $line(%w,$calc(%l - 1))),$chr(9))
          dline %w $calc(%l - 1)
          dline %w $calc(%l - 1)
          dec %l 2
          var %com 0
        }
      }
      rline %w %l $str($chr(9),%tab) $+ $line(%w,%l)
      if ($regex($line(%w,%l),/\173$/)) {
        ;inc %tab
        var %com 0
        var %temp $true
      }
    }
    inc %l
    if (!%temp) inc %com
  }
  write -c %f
  savebuf %w %f
  window -c %w
}