How do you re-instantiate an iRule after external Data Group changes?

To answer my own question: You do not have to re-instantiate the iRule, you can simply cause the external data class file to be updated. This can be done by updating a property of the class definition in bigip.conf .

For instance, if you issue a:

b class Test_Mass_Redirect_Datagroup mode rw 

Even if the mode is already rw, it will update and re-build the hash table.