Some people use up and down arrows above standard staff notation to indicate whether you blow or suck. Is it so much harder to read notes on a staff than to count hole numbers? - it's not like you can actually read the numbers while you're playing.

Suggested convention: ABC already supports violin up- and down-bow signs, so any reasonable implementation will let you print them. None that I know of will let you add arrows. So, why not use the up-bow sign for suck and the down-bow sign for blow? That makes it very easy for people teaching the harmonica to use existing ABC transcriptions.