Question BlendOp LogicalOr with DrawRenderers Custom Pass?

I'm trying to use a DrawRenderers custom pass to select/identify objects on a Custom buffer to then pass to and use in a second FullScreen effect. I'd like this buffer to take the form of a bitmask so I can identify multiple overlapping objects belonging to different groups in the same pixel. In the DrawRenderers surface shader I tried using the LogicalOr BlendOp to build the bitmask buffer, but it doesn't seem to be working. I've noticed that the docs mention that this doesn't work on sRGB, bit I don't see if there's any way to configure that for the Custom buffer, and I'm not even sure that's the problem here. Any thoughts on how I might accomplish what I need here? Open to suggestions and weird workarounds.

Here's an illustration of where I'm at and what I'm trying to accomplish: