Display registered sidebars in Admin based on blog (using one theme)?

I’m using the multisite functionality. All of the blogs in my installation are using the same theme.

I have registered several sidebar regions in the ‘functions.php’ file. Because of this, all of the sidebar regions display in the WordPress Admin (Widgets screen) no matter which blog is being edited in the Admin.

However, not all of the blogs use all of the sidebar regions. In other words, I only want certain sidebar regions to display in the Admin based on the blog being edited.

Is it possible to register select sidebar regions based on the blog id? Or do all of the sidebar regions need to be registered for all sites using the same theme?