With a sump you would be better off making a traditional reactor or mist, both of which will not be exposed. Either of the 2 can get the Co2 levels quick enough and maintain it since you will outgass constantly.
What you are suggesting/thinking is going to cause problems for if you want to grow healthy plants and without algae drama later on.
Okay I like keeping it simple I like it!! So what do you mean by traditional reactor or mist??