And to answer the question of why you would want to do it after the display is created... you might want to optionally "lock" the windows in such a way that as the user moves the main window, the toolbar windows are moved with it. This is particularly useful perhaps with a frameless window.
I think it's silly to artificially restrict Allegro just because we cannot think of a reason today of why something would be useful. Sooner or later somebody will stop by with a reason to need to change the constraints on the fly (etc).
I don't have a problem with constraints being part of window display options, as long as it can be set after the window is created as well. We already have a al_toggle_display_flag()... we basically just need an al_set_display_option() function as well.
But that said, I'd rather it not be a display option because I don't see any reason for it to be. Unlike things like "frameless" or window positions, the user isn't going to notice if the display is created and then subsequently constrained to a certain size.