And how about adding a built-in enable/disable system for mods? This will help also when getting errors that cause serious problems to the forum; if SMF notices that something doens't work properly, automatically (or prompting an advice that invites to) disables that mod (like wordpress, that imho has one of the best plugins system)! 
Don't know if it would be too difficult to add, maybe for the next major release... 
There are two ways (AFAIK) to make software plugins:
- Either allow mods to modify code inside the original software. Best flexibility, but may cause conflicts with different versions or additional mods,
- Add "entry points", "hooks" into the original software code, that will call a series of functions that belong to the mod. This would allow such a feature as what you're referring to, but would make mods much more complicated to develop, if only because hooks can't be placed everywhere (it would kill performance), so there are many things you couldn't do.
So... I guess they will tell you no
