How to apply a PostProcessingVolume bloom to only top camera / single layer?

I want a bloom effect on my semi-transparent UI, but not on the rest of the map layers beneath. Is this possible?

I have 2 cameras:

• UI Camera on top (depth 1) on UI Layer, Culling Mask: only UI layer, Clear Flags: Don't Clear

• Map Camera underneath (depth 0) on Default Layer, Culling Mask: everything but UI layer

• PostProcessingVolume and PostProcessingLayer on UICamera.

• PPL Trigger points to UICamera PPV (this)

• PPL Layer is set to UI only

• PPV IsGlobal is checked

I get the feeling that PPV.IsGlobal and PPL.Layer are ignored by the bloom effect... but can't find any documentation to prove it. By all accounts it looks like this new (somewhat obtuse, poorly documented) PostProcessing setup is designed to do exactly what I want: apply an affect only to one camera or layer. But I've twiddled all the settings and either get bloom on everything or bloom on nothing.