I had a need to use an alternative BB Themer Layout assigned to a location if a query parameter was provided and matched a predefined value.
I originally hacked up a solution using fl_theme_builder_current_page_layouts and iterating the $layouts object to re-assign a new themer layout, but that seemed pretty hacky, so I was happy to see the new Rules/Conditional Logic API introduced in BB Themer 1.2
Here is a starting point that works with the query parameter bbtl based on BB's example code. Just choose the rule, your operator, and assign the value.
Have fun.