I was interested in allowing custom High Valyrian houses and Targaryen cadet houses the same coin flip events, so I wrote a small mod that does just that.
It should trigger if you are a High Valyrian and haven't been prompted about your culture (ie. through the character creator)
This mod modifies the scripted triggers "is_divine_blood_dynasty_trigger" and "high_valyrian_dynasty_trigger" to also include "has_dynasty_flag =targaryen_blood_custom". It also adds a new scripted trigger "targaryen_blood_trigger" which triggers for Targaryens, Blackfyres, Bittersteels, 'of Essos', the four cadet dynasties, or if the dynasty has the above flag.
I've replaced all the references to the four named houses AND the four named houses with the four cadet branches in the high_valyrian_events file to not look for the dynasties, but instead look for targaryen_blood_trigger=yes (this includes events relating to the coin flip, wildfire drinking, embracing the Valyrian religion, and Dragon Dreams).
The flag "targaryen_blood_custom" can only be assigned with the event "high_valyrian.1" (the one which fires for new high valyrian characters). There are now two options for the player: the standard "we're High Valyrian" vanilla option, or the "we're High Valyriand and subject to greatness", ie. related to the Targaryens.