Can you please explain why you want to override this pyGuid property/column ? It's not recommended to do this, and the template runtime internals always set this. We can evaluate if there is a product gap that makes you think you want to override pyGuid.
We want the pyGUID column to be in sync with a column in another user defined table.
When the campaign writes into this table, we also want to persist a subset of data(includes pyGUID) into another table.
We would not know the value of pyGUID until its persisted into the first table. So, I am trying to set a value (using generateInteractionID()) and trying to see if we can set in both tables. But, since its a 'py' column, output template does not allow me to set the result of generateInteraction().
Note- we are not leaning towards creating a new identifier in the table and index them.