Custom Exception & get_instance()

Hello guys,

i was searching for hours to find a solution to generate usable 404 errors in CI, but i think this section is not well integrated in CI:

When extending the CI Exception class, there is no way to get the instance with "get_instance()" because as what i've seen, CI isn't loading the core before the Exception controller. 
I need some basic functions to generate my 404 error pages, so this behavious is completely useless for me as it only handles static output.

If i use the method with 404_override in the config file, it works well, i can use the whole CI core functions but it is also not thought to end because sometimes i need to call show_404() manually which shows the ugly default error from the CI Exception class which leads to the problems above.

So there are 2 ways of integrating, but none of them is really usable. 
Sounds like a joke to me...

Has anyone solved this problem without modifying the core? 

Thanks in advance,
Daniel