network distributed storage with windows?

[A Rossi]

Hi,
I've been hired by a client to perform a number of network services for 
him, most of which are completely unrelated to my topic.
Now, onto my topic:
He asked me if I could partition all of his workstation computers 
(running windows XP Professional SP2) with a windows partition, and a 
hidden partition which occupies most of the disk, that is accessible 
over the network to OpenBSD (actually he asked for FreeBSD, but I will 
change his mind...) to back up his server. He doesn't want his employees 
to know about it or to be able to interact with this "hidden partition" 
in any way. I told him that it is not possible, because windows controls 
the hardware (being the OS on the system) and the only way it would work 
was if he had *BSD on the system. But, because he is paying me, I 
thought I should give him the benefit of the doubt, and ask the pros in 
this area.
So, is it possible for OpenBSD to access a bunch of "hidden" (I put it 
into quotes because it could be any non-windows compatible partition, 
because it won't show it then) partitions on networked workgroup 
computers and treat them like one big disk for backup?

My apologies for such a long post. I am new to OpenBSD, but I like what 
I see.
Thanks,
A Rossi