Feature Request - Specify which layer to auto align to

My idea for a feature that hopefully would be simple to integrate is to have the ability to specify which layer is the anchor layer when you are auto aligning layers. A lot of times you have images from photogs that didn't shot on a tripod (or whatever) but you need the bottom layer (or a specific layer) to be locked down  and would be useful to have the stacked layers above modify themselves with auto align to the bottom anchor layer (maybe have a box that says Use: Layer XYZ as Anchor layer or something similar.

Just trying to make this program even better. 

Thanks for your time.