OK, here's my take. You won't get skewed results from placing it any where between the filter outlet and the reactor. However, not knowing how the probe holder is engineered, you might want to mount it so that air doesn't get trapped around the probe.
I've worked with inline probes for 15 years in biotech manufacturing, mainly in the purification of proteins, and I know that inline probes are used mainly for monitoring trending data, meaning, for real-time analysis of specific operations. They're not used for processing decisions, i.e., for controlling other equipment to control the parameters of the fluid in the piping.
When probes are used to control a pH change in a tank containing a cell culture or dissolved protein, the probe is always in the tank, where it can take a broader sample and react more quickly to the whole of the solution rather than a small sample of the whole going through a narrow tube. We also use probes without the plastic sleeve, so that flow is laminar around the probe tip.
Depending on how large your tank is, my concern is that the inline probe will react more slowly given that it's not sampling the whole tank, so it will be calling for more CO2 when the setpoint may already have been reached in the tank. Resulting in overshooting the desired pH setpoint. I guess it also depends on how fast your CO2 flow rate is and big your reactor is.
It's worth a try, definitely, but you might need to tweek the system a bit to get the control you want. It would be good if you can mount the probe holder so that you can pivot the tubing to get any bubbles caught in the tip of the probe out. Also, how do you intend to check and maintain your probe once it's installed?