Um, I suppose if you wanted to actually put a windows window inside the a5 display, you can get your hands on the HWND (see: al_get_win_window_handle)...
I'm not sure I'd do it that way, depends on the API you have to use.
The two most likely ways I'd try are:
1. Find the windows api that gives you raw frame data from the hdmi input, and then upload that to the a5 display (al_lock_bitmap... etc)
2. Find the windows api that lets you give it a HWND for it to display to, and hand it the a5 HWND.
The third alternative would be if the API you get only gives you a HWND for some reason, then you can make it a child of the a5 HWND.