I think that would be an incredibly difficult task to accomplish. Currently these viral methods require a precise injection to a very specific part of the brain

I think that would be an incredibly difficult task to accomplish. Currently these viral methods require a precise injection to a very specific part of the brain depending on what you're trying to modify or study, it's not like you can just give someone a pill that does this stuff. In optogenetics, light needs to be delivered inside the brain or to the neurons a brain slice or culture. That's on top of the fact that many behaviors are highly complex involving numerous circuits. You can already give someone drugs or take a drug yourself to modify behaviors and we get greater specificity with newer generation drugs as we learn more about the affinity of ligands with subunits expressed in various circuits naturally. Modification of behavior and thought processes is already a reality and it is in the field of psychological and behavioral neuropharmacology where we study these methods to help people with mental illnesses and neurological abnormalities