@selea Your most likely candidate for this would be NGINX. https://aimechanics.tech/2020/06/12/ssh-forward-with-nginx/
@selea I think given the right setup for NGINX it's possible but I also think that there might be an easier solution to the issue your facing. Could you not just setup the SSH config so depending on where you want to SSH to it uses different keys? Why is there a requirement to use keys rather than hosts?
@joshfowler
I am not sure it can proxy ssh depending on the public key used, but nice to know anyway :D