I want to create an object that acts like a lens, and when you look through it the render pipeline is altered.
For example, a window that when you look through it, the other side goes from cartoon to realistic.
I know that is possible with shaders alone but if modifying a pipeline is possible that could be a lot simpler.
Is that possible?
I think you want to look at render textures, not render pipeline. The only way to alter a render pipeline is in C# - so the answer is no.
But typically a magnifying glass or lens effect is done with glass shaders, or copying the rendered display to a texture and rendering a mesh.
Look for glass shaders and similar. Already built into HDRP. Again though, you dont need to or want to alter the “pipeline”.
If you used HDRP, you could implement this as a custom pass, if the effect is not something that can be achieved with the standard HDRP shaders.
I think it might be a nice use of stencils and possible in Universal via it’s new forward system. I’m still not sure you would need to alter the pipeline itself though.