If I was in any way skilled with acrylic....I'd try my hand at
building a prototype.

I am and have built them but they would run 100$ each and would take a
great deal of labor to produce.
You could speed up production etc but would you sell enough to warrant
that?

It's business, you need to sell a large number of filters to make that
profitable.

A simple add on tube with it's own pump so that the aquarist does not
need a solenoid to have semi or automatic control is a more flexible
better idea. The added flow rate with the extra pump increases mixing
of CO2 when the plants need it.

Filters get clogged and slow down, get hair and what not in them. This
will effect the CO2 also.
By having a separate CO2 system, you avoid this issue.

If you have a fluval or a HOT etc, you can add a CO2 reacting tube
after the filter and some have done it before. Check valves and
solenoids are needed for semi or automatic control, but if you add CO2
24/7 , it's less of an issue.

If you have a hang on the back wet/dry filter , you can place the CO2
reactor in the sump or in regular wet/dry sets up as well.

That's what I do.
Regards,
Tom Barr