Show User Profile Field only to certain usergroup?

[is mise]

Is there a way to make a user profile field show up only to members of a certain usergroup?

Here's my problem:

I have several public forums and one private forum. The private forum is only visible by members of a particular usergroup. These people know each other in person ("real life"), so we want them to be able to figure out their real names.

Regardless of what name they register under (CoolDude, GreatGal, whatever), I want users in the private forum to be able to see their real first name. I figured I could let them put their real first name in a custom User Profile Field, but then everyone (in the public & private forums) would see their real name.

Then I thought maybe I could set permissions or something to allow only members of the private forum/usergroup to see the FirstName user profile field.

Is there any way to do that?

Thanks!

P.S.--I bought that new vBulletin book from PACKT publishing, and it doesn't seem to discuss this at all.