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 sort of thing is not allowed according to the CEC and should also not be tolerated by inspectors either.

A panel enclosure shal not be use as a wireway or raceway. something to that effect is in the CEC.

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.