I'm by no means an expert here. I do have both a pressurized CO2 planted tank and a turtle tank, but they are physically separate aquariums. Any equipment in the tank the turtle can chew on or mess with. I would go for an inline diffuser or inline reactor on the return side of your canister. If we can keep fish in a planted aquarium that breath dissolved oxygen by passing water over their gills, then having a turtle that breathes air shouldn't be affected by the increased CO2 level in the the water. As for the PH, here is the classic chart showing CO2, KH, and PH relationships. It is possible to have a proper CO2 level for optimal plant growth at a PH level that is normal to a turtle.

What WOULD worry me is the fertilizers. The turtle would be just fine with increased lighting. Adding dissolved CO2 should be OK as well. However when you add lighting and CO2, you also need to add fertilizer. These can be toxic to the turtle in high enough concentrations. I unfortunately don't have any research or data to back up my thoughts but if I were you and you're trying to combine a planted with a turtle tank.... then I would focus on how safe are the fertilizers to have in the water column with the turtle who drinks that water.