Before rendering the screen layout harness, pyDefault data transform in Data-Portal class will be invoked in pyDisplayHarness page context.
This can be used to initialize properties prior to portal load.
Apart from this, we also have a extension point PreDisplayHarness Activity (with Applies to @baseclass), this can be overridden into application ruleset for any custom implementation. Again this will also be invoked in pyDisplayHarness page context.
I know its a delayed response. But it might be useful to some one else. Harness name is passed as parameter to this page. Check for the harness name in the parameter using when condition and run your steps only if the harness name matches else skip the steps.