Your plan certainly won't hurt, and will probably help, but you don't want hermit crab poo building up any more than you want octop poo to. What about letting water flow do most of the work? What if you fed a hose into the carrier, through a tight fitting hole, and continuously ran a small powerhead outside the carrier, connected to the hose,? The idea is to continuously blow "clean" tank water into the carrier at a rate that is just fast enough to pickup and keep particles of poo and small food scraps suspended in the water, where they will eventually flow out of the carrier. Any pieces that are too big and heavy can be eaten by either your octopus or hermit crab, and the hermit crab poo should be small enough to blow away and not collect in the carrier. The environment and water chemistry in the carrier would be as clean and stable as in the big tank, and your little octo wouldn't have to endure daily alien abduction or the subsequent PTSD. If the smallest powerhead you can find creates too much flow, you can drill small holes in the tubing, before it enters the carrier, which will let some of the water, and pressure, escape before it goes into the carrier. Drill as many as you need to knock the flow down below gale force, but still fast enough to kick up all the poo so nothing can settle on the bottom.
If you decide to drill a hole in the carrier, or its lid, to get the hose in, be sure to back up the thin plastic with a flat piece of junk wood when you drill, so that the drill bit pushes through the plastic and immediately into the wood. Press the wood and plastic tightly together so neither one can shift or spin during the drilling. If you don't, the plastic is much more likely to crack, or chip during drilling. I suspect that sandwiching the plastic wall of the carrier, or lid, tightly between two pieces of wood would be even better, especially for thin plastic. This trick has worked very well for me when drilling 1/8" - 3/16" acrylic.