Well, you could create a membergroup called Moderator. The way I did a title change like that so it was more visible (bold and different color) is do it through the table. It's kinda sloppy, and probably bad, but it worked fine for me. You can try this:
Go into phpMyAdmin, navigate to the table yabbse_members
Click the Browse option so the member records are displayed.
Find your user, and click the edit next to his/her name
Find the row named membergroup. In the large text box, you can put whatever you like, such as: <b>Moderator</b>, which would show
Moderator by his name.
Scroll down to the bottom. The radio buttons 'Save' and 'Go back to previous page' should be selected, then click go.
That's it. Sloppy, yes. Works, yes. Stupid? Only people who know more than I can tell me.
However, this will display moderator on his name everywhere, even though he'll only be able to moderate the boards you assign him to.
Methonis