Hey there @openbridge-james! Unfortunately, the event.organization object is currently only available in the Post Login Action - I definitely recommend creating a feedback request for this if you are up for it. I did a quick search and it looks like this hasn’t been added previously, but is a common ask. Our product team monitors these requests closely and the request makes total sense to me 
As an aside, it looks like the data you need may be available via /api/v2/users/{id}/organizations at the time of the post user registration action/flow.