Ask and ye shall receive. Not as a rumor, but as something you can buy today (albeit just barely under $500, by $1).
Buy Canon RF 100-400mm f/5.6-8 IS USM Lens featuring RF-Mount Lens/Full-Frame Format, Aperture Range: f/5.6 to f/45, One UD Element, One Aspherical Element, Super Spectra Coating, Nano USM AF Motor, Maximum Magnification: 0.41x at 400mm, Optical Image Stabilizer, Customizable Control Ring...
www.bhphotovideo.com
Note that with longer lenses, there is no design advantage of a smaller image circle because the size of the image circle is not limiting with those lenses, rather the entrance pupil diameter (functionally equivalent to the front element in a tele design) is the limitation. That's why you don't see Canon APS-C lenses longer than 200/250mm, it's not more expensive to make them to cover a FF sensor.