I can add a button that unmaps ASUI and then lock&blank screen but leave keys unlocked. Whatever app is on top of the window stack would then receive the keys. Pressing the volume keys would work fine but if any other keys were pressed it could switch out of the app or put it in a state were volume keys don't work. Another option would be to leave ASUI mapped but in an audio mode and let it handle the volume while ignoring the other keys or using them for various functions such as mute for fullscreen. If the audio apps respond to dbus signals I could have the keys map to functions like start/stop, next/prev, etc.

But first I must fix the dpad center problem. If I understand this correctly, pressing any hardware key, even dpad center, causes the "press dpad" notification to appear, but ASUI is displaying that message without receiving a KeyRelease event which actually displays the message. But setting fullscreen as the secondary key works fine?

If I could get you guys to download the newest debug binary asui dd;asui id, and then perform the following tests and send me the log.

Set dpad center as secondary key: lock it, press each key once.
Set fullscreen as secondary key: lock it, press each key once with fullscreen last.

This build will show me exactly why dpad center isn't working.