Well I don't believe mIRC has any builtin way to reverse text, but making an alias to do so is pretty easy:
alias reverse {
var %txt
var %i = 1
while (%i <= $len($1-)) {
%txt = $mid($1-,%i,1) $+ %txt
inc %i
}
return %txt
}
$reverse(this is a test) = tset a si siht
$reverse(tset a si siht) = this is a test