See T9299 for some context and discussion.
An immediately achievable half measure here is:
- Create accounts on the instance from a big list which mostly overlaps with users who are likely to get invited.
- Link their emails to the instance on admin, but don't complete the invite workflow (no account creation, no invite email).
This is basically analogous to a normal import, we just stop halfway. When users sign up and verify their email address, they'll be linked to the instance automatically and when they arrive on the instance their account will already be associated with pending reviews, etc.