Send custom signup approval email to different Administrators selectively

I have the usual sign up process:

1. User completes registration form successfully
2. User receives activation email
3. User clicks activation link and activates the account
4. Administrator receives approval request email
5. Administrator approves (or rejects) user
6. User gets final approval email

Question: At step 4 in this process I want to send the approval email to only one of several Administrators conditionally depending on a field from the registration form.

I'm looking for advice on the best way to do this, any potential issues? If there are plugins that achieve this, great, otherwise expecting to build my own - any advice/experience on relevant API calls appreciated.

Background:

• The reason for this need is the organisation has several departments, and we want to get away from keeping a centralised list of who is allowed to sign up from each department held by a single Administrator. Instead we want to send the user signup approval to an Administrator in each department who has access to the necessary information
• In case it's relevant it's a site that makes heavy use of Buddypress; the field that will determine which Administrator the request goes to is a Buddypress xprofile field.

Thanks

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