Two pieces, built as two separate Android modules:

1. qs-tile-app/ — a tiny app with one Quick Settings tile. Tapping it flips a persisted system property with root: persist.lunaris.btmicsplit = 1 (on) / 0 (off).
2. xposed-module/ — an LSPosed (Zygisk) module. It hooks the calls apps use to request a Bluetooth SCO link (AudioManager.startBluetoothSco(), AudioManager.startBluetoothScoVirtualCall(), BluetoothHeadset.startScoUsingVirtualVoiceCall()) and no-ops them whenever the property is 1. No SCO link ever comes up, so AudioPolicyManager falls back to the built-in mic for capture — while A2DP media output keeps routing to the BT earphones untouched, since that's a completely separate audio path from SCO.

Nothing here patches the audio HAL or AudioPolicyManager itself. It's a pure app-layer choke point, which is why it's portable across ROMs — you need LSPosed (Zygisk-based, e.g. via Magisk + the LSPosed zygisk module) installed, and root for the QS tile to set the property.

Both are standard Gradle Android modules — open each folder in Android Studio (or add as modules to one project with a shared root settings.gradle), build the APKs normally. Nothing exotic in dependencies beyond the Xposed API (de.robv.android.xposed:api:82, compileOnly — matches what LSPosed implements) for the hook module.

1. Install xposed-module APK, enable it in LSPosed Manager, scope = all apps (the "system framework"/system_server scope is not enough here — the hook target methods run in the calling app's own process, not system_server, so scope needs to include whichever apps you want this to affect, or just select all).
2. Reboot (Zygisk modules apply at next zygote start).
3. Install qs-tile-app APK, grant it root when prompted, add the tile to your Quick Settings panel.

This affects VoIP/media apps that use separate AudioRecord/mic capture and media-output streams — WhatsApp/Discord/Zoom calls, voice memo apps recording while music plays, etc.

It does not split a native cellular phone call. HFP bundles TX+RX into a single SCO link by protocol design — a real phone call's mic and earpiece audio can't be routed to two different devices, root or not. If you enable the toggle during a real phone call over a BT headset, expect the call to just fall back to phone mic and phone earpiece/speaker, not phone mic + BT speaker.

Since you're already building Lunaris, the "correct" long-term home for this is a small patch in frameworks/base (AudioDeviceBroker/BtHelper, gate the same startBluetoothSco path behind a Settings.Global flag) plus a SystemUI QS tile — no LSPosed dependency, works even without Zygisk/root at boot. Happy to spec that version out instead if you want it baked into the ROM later; this standalone version was scoped as requested so it also works on stock/other ROMs.