Sam, you've got a beautiful mangrove/macro tank, so I yield to your experience. However, I'm no expert, but I think some of what you said above is incorrect. I don't mean to be the knowledge police, I just wanted to clarify or correct a few points.
CO2 is not used in FWPTs to keep pH low, it's used as a carbon source for otherwise carbon-limited plants.
CO2 is used with calcium reactors, which boosts calcium, not reduce it. It can, over time cause pH to drift lower, so some combine it with Kalkwasser. Effectively, every reefer out there using a calcium reactor is also dosing CO2. The acidity basically dissolves calcium in substrates, making it available to corals, plants, shrimp, etc.
Carbon IS in short supply in most tanks. Carbon dosing addresses this.
As you say, it's not needed for healthy macros, but I bet they would benefit from it. Having said that, would I recommend folks go buy CO2 setups for macros? No, but if you've already got the components (from a calcium reactor or freshwater plant setup), why not give it a try?
The tricky part, I think, is whether it might bother other more sensitive (and expensive) organisms in our tanks, or would it favor micros over macros.
I have used CO2 with success with my seagrasses. I have no idea whether it helps with macros, but it seems likely to me.