Alerts subscription for role members of a custom role provider

I have a SharePoint 2010 website that supports only FBA authentication/authorization through custom membership/role providers.

This provider will be used by a number of .NET applications other than my SharePoint website, and all of users management and authorizations will be done in a central application outside SharePoint and based on roles.

This scenario works fine for giving permissions, but it doesn't work with alerts. I want to enable  administrator to subscribe a particular role to email alerts, and all users of this role to receive email notifications.        
what options do I have?

Ghobary