Can you have replacement variables in one forum?
 

Can you have a certain set of replacement variables to only be active in a certain area?

Replacement variables can only be set for a specific style.

Is there a plugin available (I did search but didn't get any results) that could allow you to change certain words only in certain forum sections.

Actually, I think you may be able to accomplish what you want by making a child style of your main style and then set this child style as the only style for the forums you want to use replacement variables on. Then set the replacement variables for just that style, which will in turn effect only those forums.

I hope that made sense.