currently untested.
hyprland itself appears to lack the ability to crop a mirrored monitor. however, wl-mirror can be used to mirror and crop a monitor into a window:
$ wl-mirror -s c HDTV-1
hyprland/wayland itself appears to lack overscan compensation:
(although there are claims it can be configured via modeline. haven't tested this in hyprland yet although it does support modelines.)
however, its config allows for reserving space at each side of a monitor:
monitor = name, addreserved, top, bottom, left, right
monitor = CRT-1, addreserved, 35, 35, 40, 40
although this isn't respected by fullscreen windows.
by using these in tandum i am able to properly fit 4:3 content on my crt tv which is mirroring my 16:9 flatscreen led tv.
first i use wl-mirror -s c to create a "scaled to cover" mirror window of my 16:9 tv.
then i move that as the only window on my 4:3 crt tv
and adjust each addreserved side until the window border is just beyond the overscan.