Para referência futura, esta foi a solução:
<location path="wp-admin">
<system.webServer>
<security>
<ipSecurity allowUnlisted="false">
<clear/>
<add ipAddress="ipaddr" allowed="true" />
</ipSecurity>
</security>
</system.webServer>
</location>
<location path="wp-admin/admin-ajax.php">
<system.webServer>
<security>
<ipSecurity allowUnlisted="true">
</ipSecurity>
</security>
</system.webServer>
</location>