What is the best way to access stored procedures in Django's ORM
Don't.
Seriously.
Move the stored procedure logic into your model where it belongs.  
Putting some code in Django and some code in the database is a maintenance nightmare.  I've spent too many of my 30+ years in IT trying to clean up this kind of mess.
I am designing a fairly complex database, and know that some of my queries will be far outside the scope of Django's ORM. Has anyone integrated SP's with Django's ORM successfully? If so, what RDBMS and how did you do it?