Whilst small, direct messaging will not be an issue. But...

"250 total direct messages per day, on any and all devices"

http://help.twitter.com/forums/10711/entries/15364

Im not sure if they will make an exception for you. I too wanted to us dm as a means of communicating, but having a limit of 250 in a day would just be crippling.

Perhaps an argument in your d glu would be to allow for "trigger word or phrase" then an open message would not be as bad.

Another option would be you send a tweet with a link. The page has a password which displays the message for the user.