Can unusually deep equipment project through the rear of a 19-inch cabinet?

Yes, where the cabinet is installed with its rear open and accessible.

The rear metal cover on the 19-inch cabinets can be removed, exposing most of the rear of the internal rack. This can allow suitable equipment to extend beyond the normal enclosed cabinet depth.

This arrangement is not suitable for a cabinet mounted conventionally flat against a wall because the wall occupies the space immediately behind the cabinet.

The projecting equipment must remain properly supported, and the rear of the installation will no longer be enclosed by the cabinet.