I'm not sure I understand your post. As I understand it, "contrained to original ratio" means that if the original size is (say) 3000x2000, cropping 10% from the long side (to 2700) would automatically result in the short side becoming 1800. If by "custom ratio" you mean cropping to 3000x1000 (to create a panorama) this is exactly what I did in Photoshop; and the result (in print preview) was an image with wide white top and bottom margins when the "scale to fit" box was ticked.
Without ticking this box, the given ppi and image size resulted in an image that filled the paper, yes; but only with that part of the image that could fit unless reduced in size (i.e. centre of image only). As my "normal" files are about 30,000x24,000 pixels, I have this box ticked by default. And I always get wide borders if the image proportions don't match the paper proportions.
As I understand the comments in the Canon printer driver, it's possible to overrule the wide borders at the printing stage, and stretch the height to again fill the paper, eliminating the wide margins but distorting the image. Not having the printer, I can't confirm this from experience.