"They do pretty good in brackish water also, about 1.090-1.013 or so. I've grown them in (w/seal bottoms)pots with some soil and sand and some brackish water (enough to cover the bottom inch or so of the stem) and added a jobe's stick in every so often(every 3-4 months)."
Technically, they would do just fine in full fresh water in well fertilized soil. Rhizophora is not a shoreline dweller because it can't survive in favorable conditions, it is a shoreline dweller because it is competitively inferior, and adapted to cope with the water stressed shoreline (exclusion, osmotica, aerenchyma, etc.). As adaptivity/tolerance increases, competitiveness always decreases, and vice versa.
Increasing CO2 concentrations of the water would not help unless you are agressively venting CO2 (ie, aeration, powerhead) at the site of the mangrove, as they take up CO2 from leaf stomata and lenticels (pores in the trunk and aerial roots for gas exchange).