That wouldn't work, mIRC parses the line first tokenizing by : ( i assume)

on $*:TEXT:m/:/:#:echo -a message: $1-
would mess up the token count.

so even if you did
on $*:TEXT:m@:@:#:echo -a message: $1-
It would break it.

a simple alternative is to use \072 for :
on $*:TEXT:/\072/:#:echo -a message: $1-

or if your really concerned about backreferencing

on $*:TEXT:$(/(? $+ $chr(58) $+ a){3}(b)/):#:echo -a message: $1- regml: $regml(0) $regml(1)