Actually in our current implementation we are adding a couple of pages to pyDisplayHarness and using it across screens.. So it really recommended to use pyDisplayHarness for application specific activities
Firstly, I am not sure about the significane of pyPortalHarness page !
Second thing , regarding the customization of pyDisplayHarnessPage, I think it is not recommended to play around with it as this holds the key of opening any harness ( be it ruleform or portal ) . But if it is something that you have to access and twig then you can try that. There is nothing wrong trying something ! If you face challenge then you can get back to PSC