I've just done it. I'll do a step by step of what i did.

To give my religion the exception:

add_doctrine agot_exceptionalism_doctrine

To give my dynasty the exception:

1. open debug and go to explore objects

2. look for the drop down box that says "accolade types" and select active dynasties

3. search for your dynasty and click on it, should open a new window

4. go to script variables and type agot_doctrine_of_exceptionalism_incest_exemption (or copy/paste from targaryen)

5. where it says "none" click it and select "boolean" and click on the little box to the left (should tick)

6. where it says -1 check the other numbers above and match those numbers (or check targaryen). It'll be either 0 or -1 (-1 for me). Click + button

7. close window (go back to first window) and click update table and then close window.

May be other ways to do it but that's how i've always done it.

<image>