In my experience, iguanas do not like to get poop on themselves. Maybe there is an environmental stressor that causes him to act out. Cage too small, too hot, no cool zone, too low to the floor? It's a guessing game.
I can walk you through the construction of a "shower pan" for the viv. With the bare plastic as floor, you can set up a pump to automatically wash the feces down the drain. I have a ton of information in residential construction and remodeling. I see this shower pan option very clearly.
First, do you think it's possible to train him to be tub-pooped before you go to work? I think he can learn this within a month of short-session tubbing. even if at first he doesn't like it. No doubt you already know how to do that. The before-work tubbing is widely praised as the simplest strategy for coping with an ig's bowel habits.
I hope some of this helps.
I do like your idea.
The answer to your question seems to depend on how large your own viv's footprint dimensions are.
In a bio-viv, your ig will probably still poop-paint.
I found several instructional web sites/forums devoted to this topic. Could you find an on-line forum on this subject? I think they are there. That's where the experts are.
Plus, your ig is very likely to eat the substrate, the living plants, and the leaf litter. You can't allow that.
From what little I've read, biovivs need to digest all biological products that are introduced into the system, completely, every day. In your case, the iguana food outside the system is introduced through your ig's feces. It seems to me the specific bioviv you want must convert your ig's feces into compost, completely, every day, or else it will build up into a mess.
IMO the substrate will need to be 24 inches deep, maybe more, to get much composting work done. It seems to me that the feces of the day will have to be pulled into pieces and buried in layers within the viv to speed up the conversion. So your viv will not be totally self-sufficient. It will need attention every day.
This bioviv may even need to be big, like 3'X 10 ft. or more, to try to get the job done.
Even a very large iguana viv has an only 3' deep X 8 ft long footprint, giving only 24 square feet of space for a self sustaining composting eco-system. Almost all iguana vivs are quite a bit smaller than that.
I can't imagine how flowing sewage will be created. The scat will sit on top of the soil. It has to be buried in the dirt so it can decompose. Plus, if some of this gets into the coarse-rock bottom water storage layer (you could call this zone "The Sump"), who can predict what will happen then. The water has to be healthy so it can be pumped through a filter, then into misting nozzels within the viv for your ig to breathe in, without causing your ig to get sick.
This is all merely supposition, based on what little I already know. I hope you experiment with this, so you can share the findings here. Maybe make a planted viv without any animal larger than one house gecko, to see how it performs.