Flirting with the stigma of that play, are we insel?
Here's a brave artistic director who named his ensemble The Defying MacBeth Theatre Company just to challenge the superstition. here he gives an interesting look at the superstition's legend:

http://www.angelfire.com/fl3/Defymcbeth/Super2.html

As for safely alluding to that character...can't seem to find or recall a definite euphemism, but it seems I've heard Shakespeare's Scottish general or that Scottish general, or even that husband of Lady M.

There's also a detailed and intriguing look at the superstition at this site, which contains this once-recommended ritual for actors who uttered the unmmentionable:

Any actor using the "M" word in a dressing room "should immediately leave the room, turn around three times, break wind or spit, knock on the door and ask permission to re-enter. Alternatively, (and less cumbersomely) the line "Angels and ministers of grace defend us," (Hamlet 1.iv) can be quoted." (Cassell's Companion To Theatre, 1997)

http://www.lyttonplayers.co.uk/Previous Productions/Macbeth/macbeth.htm