You can easily do this by padding with $chr(160) (which is now guaranteed to be U+00A0 non-breaking-space). mIRC is likely not using tabstops here either, but rather padding with spaces instead, so this will actually be the same method mIRC uses (but with nbsp instead of soft spaces).

Code:
alias rpad { return $1 $+ $str($chr(160),$calc($2 - $len($1))) }

//echo -a $rpad(hostname, 40) ; pads to 40 characters


To make sure you're padding properly, find the longest string and pad to that length (+ some extra space).

- argv[0] on EFnet #mIRC
- "Life is a pointer to an integer without a cast"