There are two questions that I see here.
Does Entra External ID for Customers support SCIM into the Entra External Id directory?
No, and there is no public ETA for when this may become available.
I.e. if an account has been created in the EEID4C tenant via API, can a user flow detect this and prompt the user to specify a password at the first sign-in?
This seems to be more of a general EEID question as there are other mechanisms for users to be created with, such as MS Graph API. I believe the answer is yes, but I'm not an expert on EEID. I'd recommend taking a look at the EEID documentation (https://learn.microsoft.com/en-us/entra/external-id/external-identities-overview), or someone else may be able to answer that second question here on Microsoft Q&A.