Alright, let me take a stab at this. I actually am a scientist and spend a lot of my time binding proteins to latex beads, which is essentially what you're asking about.
As a lot of people have pointed out, the breakdown of ammonia into nitrate is likely a multi-step process and requires more than just two enzymes. The isolation and mass-production of these enzymes would be a very costly process if it were only used in the aquarium hobby. So even if a company did start production on these enzymes, you would probably end up paying through the nose for them.
But that's not the worst part. It's certainly possible to bind enzymes to plastic surfaces. You could even toss these plastics into your filter. Unfortunately, as you said, enzymes are most stable at a specific pH. It's unlikely that the pH of the water is the same as the internal pH of the bacterium the enzyme comes from. This means it will degrade quickly. The water in your aquarium is also full of products from other bacteria that will degrade the enzymes, namely proteases. Bacteria also often have proteases on their surfaces. When they start to colonize the pieces of enzyme-coated plastic in your filter, they will destroy the enzymes as well.
In my work with protein-coated beads, I have to keep the beads sterile and at 4 degrees Celsius in order to prevent bacteria and proteases from breaking them down. Even then, they only last for 1-2 months at the most. Free enzymes in an aquarium would last much less longer, likely on the order of weeks if not less.
Nonetheless, it's a really cool idea! Maybe with enough ingenuity, someone like yourself will make it happen. But as it stands, there's a lot of hurdles to skipping the bacterial colonization step in filters. Still a great idea though; keep up with the science!
