Render ONLY the alpha pass for an animation

I have a series of product animations that have already been rendered out, but the client wants to change the distant backdrop, and I’m wanting to export an alpha pass (with motion blur) of the animated foreground object. I’m new to Blender (but not 3D), and didn’t have time or know-how to set my initial renders up carefully. I know better, now, but don’t have the time or budget to re-render entirely, and I’m still having trouble getting Blender to render a single pass only.

I’ve tried using the File Output compositing node to save out just the alpha channel on one object after removing all other lights and objects in the scene. Blender still tries to render the full RGBA image.

When that didn’t work correctly, I tried texturing the object with a flat white shader and messed with all kinds of settings, but render times seemed unusually large.

Admittedly, I’m a newbie Blender-user and I know I should have done things differently in the first place, but I’m hoping there’s a real simple compositing node trick that I’m just missing or improperly implementing.

Have you tried view layer/material override, and do a holdout material to create a mask file?
I have no idea really.

Update: It ended up being most efficient to just re-render quickly on a cloud-based render-farm in multi-layer EXR’s with Cryptomatte and just not worry about it.