Here I am. Mutinies are checked during the social tests procedure for all nations, so it's normal at least such a generic procedure is hard coded. There's no moddable event for that, only some parameters in GameRules CSV file.
For what concerns the hits inflicted to corps, the procedure works something like what follows: each nation performs some mutiny tests (the more unstable, the more tests), every time a test entails a mutiny, each army of that nation undergoes a mutiny check: the number of losses is determined by a die roll (up to a maximum as set by MME) modified by the LA3 if the general and/or commander in chief has the "mutiny bonus". Then losses are inflicted to corps (full force corps are flipped to wonded side, wounded corps suffer a morale test).
Being quite generic, the procedure is coded, only some parameters are moddable.
For what concerns the loss of offensive capability, each mutiny outcome generates a rule for the affected nation TR_MoraleTestModifier that inflicts a malus to morale tests (mostly in combats) for 2-4 turns (I recall it's a -1 on die rolls for morale tests, not moddable, sorry). Adding a turn rule is done by the code, there's no need to create a generic event script.
On the other side, you (modders) are allowed to create specific events containing scripts to generate a morale penalty to nations (use the Event effect EE_NewTurnRule to create a TR_MoraleTestModifier) or to cause desertions (use Event effect EE_Desertions), due to any historical event.