There's no reason for /echo to implement this behaviour because then a) people would request a similar switch for all other commands ("how do i /msg on a specific network?") and b) because mIRC's implementation would likely use /scon internally anyway, so it doesn't matter. The general solution of /scid and /scon is much more powerful:
If you know the cid or connection number:
/scon N echo -s hello, where N is the connection number
or /scid N echo -s hello (where N is the cid number)
If you don't know the cid or connection number:
/scid -a if ($network == SOMENETWORK) echo -s hello
That's how it's done. Just make sure you escape any user data with $():
//var %x = hello $(|) echo -a HACK! | scon 1 echo -s %x
vs.
//var %x = hello $(|) echo -a HACK! | scon 1 echo -s $(%x,0)