I have a Herbie overflow draining system and I'm wondering if I just need to put mesh over the emergency drain pipe only since I wouldn't think it would matter if the octo was climbing into the overflow, right? Just another hiding spot for him to hang out in I would assume.
Bimacs are clever, curious, strong, bored, and would much rather get out than "hang out". It's going to push and/or pull on whatever it can (hard) and try to squeeze through any crack or opening, so think through how you are going to secure the top, and the filtration system, against such a foe.
I've noticed most people use a wet/dry filtration system, but my sump has a refugium instead. Is that something I should change?
A reef system is dainty and spends all day converting light into a little food. Think of an octopus tank more like a heavily stocked fish-only tank (or a rugby team at a pizza place after a scrum). The waste comes in two main forms and both are big dumps rather than slow trickles. Shreds of uneaten food get into the water on feeding day, and a lot of octo poop comes at point in time after that. Your filtration system needs to try to get as much junk as possible out of the water as it can, BEFORE that junk needs to be broken down and processed by your colony of nitrofying bacteria. A physical filter (floss?) that gets cleaned or discarded often is great at getting the table scraps out of the tank (assuming you have a lot of flow to get those chunks up off the bottom and into the filter). An oversized protein skimmer also gets junk out before it needs to be broken down further.
In a cold water system the bacteria in your bio filter slow way down, so you need a lot of them (think oversized), and you need to limit feeding.
A refugium can be good at exporting nitrogen if you grow and harvest a lot of plants from it, but it works toward the end of the process. Even if it has a team of critters to eat scraps (a good thing) it's still better to get the scraps out before they need to be broken down. I'd be suspicious that a refugium would be a good thing (especially if it has a deep sand bed) but wouldn't be sufficient by itself.
Brittle stars are great in the octo tank. The bimac will leave them alone unless it's starving, and they will come out at night and look for detritus.
And some advice you probably wouldn't think to ask for: Even though they live togtether in the wild, avoid putting cowries in a bimac tank. Eventually the bimac will try to eat the cowrie, and as a defense it will produce a literally unbelieveable volume of slime so thick that it will clog your overflow like jello.