You're not wrong, Monkey. After much fiddling we eventually did this as a workaround. We didn't need any new bitmaps to do so either. However, this being a fundamental flaw in the natural functioning of the engine, we thought it had been problematic enough to bring to the front of the table.
Reaching this workaround was cryptic enough to catch us scratching our heads. Some may have been more perceptive. Our GUI works as originally desired now and for those struggling with the same headache, our solution is detailed below.
Because AGS does not currently draw alpha correctly over a bitmap in the same GUI, any graphics must be isolated into a separate GUI like so:
Our gAction gui is frequently toggled on and off depending of gameplay events. In order to bind the visibility of gActionBackdrop to gAction so that they are both toggled together we use just one line of code:
[code]gActionBackdrop.Visible = gAction.Visible;[/code]
Note: This code goes in repeatedly_execute either in the global script or, in our case, higher up the execution chain in its own script.
Then as if by magic, to the end-user it works essentially the same way.