Can you also look into this thread for configuring the access provisioning for multiple accounts?
IdentityNow Multi Account Support - Identity Security Cloud (ISC) / ISC Discussion and Questions - SailPoint Developer Community
You can specify a preference here for access requests. But for other modifications, @ts_fpatterson’s suggestion is what generally is preferred due to its simplicity. But if you explicitly need to do it in a single source, as you already mentioned, a complex before provisioning rule will be required to ensure the right account is selected for provisioning.
I would not prefer this approach as there are some considerations with what happens with ProvisioningResult object in the backend, which will need some manipulation to avoid ISC to trigger an unintended retry of provisioning. It can get tricky with ensuring all corner cases are handled in your code. But if you really had to go for it, consider the rule util APIs to fetch the intended account and reconstructing the account request with the necessary downstream use cases handled.