I heard you can do this, that is run branch circuit conductors for lighting circuit (or whatever) through the interior of a panel to connect to a timer or other control device that is nippled into the panel.
Is this correct by code or is it just tolerated by inspectors?
This is permitted by 12-3032 for rework. It is prohibited by rule 6-212. The consensus here is for a new installation no splices, (not a extension of a short wire) and no wire way use.