In case this is useful to anyone else, I was able to side-load the Android client to my Fire TV via the ADB shell and it runs excellently. I'm able to use my Windows 10 PC as a server can control the TV just as if it were a physical keyboard & mouse. Some notes: I had to install with the "-g" option to grant it all permissions, as the OS wasn't detecting any permission requirements otherwise and would just crash on start. Sample command: adb install -g com.clixon.across.android.AcrossClient_2.05.apk After starting the app, just toggle "Operation" on and quit. There's nothing else to configure on the client and the Bluetooth settings menu just crashes the app anyway. Fire TVs don't expose the mouse "circle" in most of the UI but it DOES work in apps that support it. The 4K versions don't work correctly with the old "mouse enabler" apks that were floating around. The cursor tracks wrong and causes more problems than it's worth to enable. This is also true of any physical mouse and isn't specific to Across. The "Enable mouse dragging between devices" option in Across works but can be a bit flaky, especially if the TV is on an app with no visible cursor. I recommend just using hotkey switching Fire TVs don't seem to have very fast CPUs, so mouse support may be jumpy and poor if you're doing something CPU intensive. Keyboard is mostly unaffected though. Now if only I could find a way to re-map keys around to emulate the "Home" button on the remote!
