Decal on Transparent / Disabled Mesh

I'm trying to make an invisible bridge that can be covered in paint using decals or something similar. I got decals working using a shader that uses the depth buffer but it doesn't work if the mesh it is on is transparent. I've tried disabled the mesh renderer, culling the mesh layer in camera, using a transparent shader that writes to the zbuffer but when doing so the decal does not show up.

I was thinking maybe using basic decals that attach to the mesh collider but that wouldn't look so good and I want it to wrap around as well .

Here is what I am trying to do.