Determine client of new case from within hook
The post hook does not perform as expected for cases :-) I have done some mods to do exactly what you are doing: automatically assigning a role. I have done this in 2 ways:


using the post hook on the Activity Open Case
using the caseSummary hook and checking if there is no other activity than Open Case. In this case I still allow the user to override what I have done as it would then have more activities than just Open Case


A refactor of CiviCRM Case is still on my wish list :-)