Change phrase for accessing a link in a hidden forum

[utw-Mephisto]

We have some forums for which you need to be in a usergroup.

Lets say someone is not part of the usergroup but accesses a link which is posted which refers to a thread within that (currenty hidden for him) forum, it gives an access denied message.

Is there a way to change that message without changing the message globally but only for that error produced by that forum ?

I hope you know what I mean