I'm running Plesk 8.1. Plesk defines the open_basedir restriction on a per-domain basis (at least mine does) in vhost.conf. All of my domains have vhost.conf, and each of them are restricted by it.
Overriding the restrictions via httpd.include will, so far as I know, override ALL of the vhost.conf settings - domain-specific settings that I want to keep. If I'm wrong about this please end my suffering and let me know how it is done.