Forgot to mention that is no longer required to provide the following credential property in order to manage credentials for a custom Account type:

IdentityConfigurationBuilder builder = event.getConfig();

builder
    .named("default.config")
        .stores()
            .jpa()
                .setCredentialHandlerProperty(PasswordCredentialHandler.SUPPORTED_ACCOUNT_TYPES_PROPERTY, MyUser.class)

You just need to provide at least one Account type to the store configuration as follows:

    jpa()
        .supportType(MyCustomAccount.class)

However, you still need the CredentialHandler.LOGIN_NAME_PROPERTY if your custom account have a different name for the "loginName" property.