I'm not aware of any right-angle RA-axis borescopes for polar alignment, but I wouldn't be surprised if there were a few made (perhaps for high-end &/or custom equatorial mounts). RA bore diameter and borescope aperture would be key factors. A relay lens or group might also be advantageous (again, depending on mount and scope dimensions/parameters).
Vixen's 8x50 right-angle "Polaris" finderscope -- found on older Celestron Ultima SCTs and also available as an accessory -- included a polar alignment reticle and illuminator. I believe it's now extinct. There may have been a few other finderscopes which utilized polar-alignment reticles, as well, but I don't recall any.