What you're describing isn't very clear. I'm confused about whether you're trying to draw attention to just one category, or always to the first sub-cat within a category, or whether it's to promote certain products/categories on a revolving basis.

What you've proposed will just double the amount of hits to the database, resulting in slowed performance.

I think it would be more effective for you to use a built-in tool like the Banner Manager to draw attention to individual promotions. It's already built-in to the tpl_header, so would be simply a matter of configuring the banner appropriately.
No PHP or additional database hits required.