[EDIT] (deleted comment that is no longer applicable to previous post) [END-EDIT]. Just as CO2 outgassing causes the pH to rise with no change in TA, injecting CO2 causes the pH to drop with no change in TA.
So injecting CO2 will lower the pH of the water (it forms carbonic acid which is acidic) and this can be damaging to copper pipes if the pH drop is substantial, which it very well might be if such injection is for a relatively short period of time so uses higher quantity of CO2. I wouldn't use copper in such a line unless the injection was going to be very slow so over an extended period of time.
If you know the amount of CO2 that will be injected per time, the flow rate of the water through the pipe, and some of the water parameters such as pH, TA and CYA, then I can calculate the expected drop in pH.