A small Bluetooth shutter release can pair with Android and send button or key scan codes to a camera app.
Camera apps may come preconfigured to listen for scancodes that map to special Android Keycode_Events.
Like "Camera" or "ZoomIn" or they may not. Some camera apps permit settings to redefine the purpose of certain scancodes it receives from the operating system.. Like the scancodes for the Volume keys VolumeUp and VolumeDown.
These can be repurposed as ZoomUp and ZoomDown.
The task then becomes discovering the scancodes being sent by the Bluetooth remote, and configuring the app to use the keycode_events for VolumeUp and VolumeDown as Zoom controls.
First listen for scancodes and map them into key code_events... Root access is needed.
Then an app like Enhanced Keyboard Helper need to be installed to perform the mapping.
Finally a suitable Camera app like Camera FV-5 needs to be installed an its ViewFinder controls configure to respond to Volume controls.