In my xmonad setup only one window (and thus screen) has the focus, so I would need to hit the key that switches focus to my B monitor (for me that's Mod4+e, B monitor is Mod4+q) and then Mod4+3.

True about indexed access; some other window managers have natively built in "tagged workspaces" where windows have tags and the "chat workspace" is just "all windows that have the chat tag". You can (probably? I've never used them) bind numbers to tags permanently or temporarily but you wouldn't be limited to 0-9. I never bothered trying to copy that setup in xmonad because I never used more than 5 or 7 workspaces.