You could swap out all the media for LR rubble. Essentially, by doing so, you'd be creating a mini-sump with some extra LR in it for bio-filtration.
In terms of mechanical filtration (catching detritus so you can dispose of it all together), LR rubble would be worse than all the standard pads / media. It's probably somewhat worse for bio-filtration also, since the LR rubble would have less total surface area for bacteria to populate.
In the end, it just depends what purpose you want the canister filter to have. If you need to run the filter pads, carbon, phosphate remover, etc., it's likely best run the way it was designed. If you're just looking for a way to have some more LR in your system and create a little more water movement, converting it to all LR rubble would be ok and serve a purpose. FWIW, our 90g came with 2 Eheim 2215s and I chose to DIY a sump and refugium instead of running either or both 2215s.
