I don't have an answer to this, but I too am keen to know -- it's a question I've already been asking!
On my Psion, I had a load of system-wide shortcuts set up, mostly using Ctrl+Fn modifiers (as nothing else used those together): Ctrl+Fn+W went to my (very old) snapshot of Wikipedia, Ctrl+Fn+F went to FreeCell, &c &c.
(For a long while I used a program called ExtraBars to do this; later on, I wrote my own Hotkeys program which was more limited but much smaller and faster -- I'd have made it public if anyone else had still been using a Psion by then...)
Of course, key shortcuts are much less discoverable, and harder to set up, than standard Android methods, but they do have two major advantages for power users. First, they're single-step: almost all other methods involve multiple taps/swipes/keypresses, but you can press a combination of keys in one go. And second, they're modeless: all other methods will vary depending whether you're already on the home screen (especially if an app is running whole-screen and needs a special way to go to the home screen), which page is showing, &c &c, while a key shortcut is always the same and can be used without even looking at the screen. As a result, key shortcuts are much faster in use and much more transparent -- you don't even need to interrupt your train of thought, and can easily flip-flop between two apps or zip directly to one.
In fact, I was wondering whether to suggest that to Planet as an enhancement to the AppBar, which is useful and cute but pretty limited. I think it'd be a vastly more powerful tool if supported three extra things:
- Files (and other home-screen shortcuts) as well as apps.
- Sublists/menus/drawers that open vertically up the screen and give you a further list of items.
- Key shortcuts to items.
In lieu of that, Automate is the only app I've heard of which looks like it might do key shortcuts. Its blurb lists keypresses among the vast list of building blocks, but I don't know whether they'd do what we want, nor how efficient or easy to configure it might be.