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I do have a setup with Android, Syncthing and Tasker. Whenever Tasker sees a specific text file when checking periodically, it generates a notification for me.
This gives me the possibility to notify me on all different stuff just by creating a text file whose content is the notification message.
You can use that for Android-local notifications as well as for notifications from any device with access to the Syncthing share.
HTH
If you're using the build that shares a user ID with Termux, you can use the termux-api shell commands to send a notification that way. You will probably have to re-package/sign the termux-api package (i just tried the F-Droid version, it won't install when you have the Emacs compatible termux).
Long-term, lisp dynamic module -> NDK and you need some manifest permissions in the APK
Years ago I had some experience using a dynamic language with the JNI to introspect the Java interfaces and call them. Fun times.
I can probably scaffold up some work, but only if people show me that they want it.