At this point, I'm fairly certain it's a bit late for that. Regardless of the situation, Yumi has always been the one that Kohaku relies upon the most. I don't think that he'll buy such an act for a second if Kohaku doesn't just tune us out.
In all honesty, I have half a mind to believe that Father Hino learned about Yumi's appearance from Kohaku's mind given how important she is to her and is using that instead of Kyoko, but that's also a risk we can't take.
I don't really want to give away our presence until we get the chance to meet him face-to-face, and even that yap gives him information about us. It tells him various things about what we like, dislike, and other minor things in the moment that give him insight into who we are. Again, this is my paranoia so it's likely not going to be that bad, but I don't want to take chances with this guy seeing how the entire arc's been kinda leading to this moment.