Particle shaders not visible when using multiple cameras

I use a worldspace canvas in some parts of my game. I need the canvas to be visible, even if behind other objects. To do this I've set up a 2nd camera parented to the main camera, set it's clear flags to depth only, it's culling mask to pick up only the layer the canvas is on and it's depth higher than the main cam. I've removed the canvas layer from the main cam culling mask.

This works nicely to present the canvas in front of other objects. The problem is that any objects on screen using any kind of particle shader (including vertex lit) disappear. If I change the 2nd cam rendering path from deferred to anything else they appear again, but I want to use deferred as it gives the best visual appearance for my canvas.

Any ideas how I can use particle shaders but still have the 2nd cam use deferred rendering? Thanks.