Sony was the first to announce that you could obtain custom frame lines in the viewfinder for a small wad of Benjamins, now it's Canon's turn, though slightly less custom.
R7, R10, and R50 users can pay US$120 to have a Canon Service Center add cropping guides to their cameras. As you can probably tell from the examples Canon shows (above), these are designed to provide you specific body/head positions for 8x10 portrait prints. You get four guides, each of which is selected with a new menu item after you send your camera into Canon for the "addition."
If you want all your portraits to look the same, I suppose this is useful. If you're doing enough portraits to justify needing such a guide, you could accomplish the same thing by simply using (near) fixed positions for the subject and camera and a marker on one of those cheap add-on LCD protectors for a lot less money. I also think it's ironic that Canon offers this with three lower cost APS-C bodies, but not the full frame ones, and that the guide cost is actually 15% of the lowest cost body you can do this with. 15% extra to add one convenience feature. Yikes.
Nikon users, are you next? Are you ready for an extra cost set of gridlines?
