Well, after long and intensive research with plenty of trial and error, I had to give up on this route. It just does not seem possible to mount a NFS Share in an empty folder in WinXP Pro. It would have been beautiful: Have one Folder C:\IMAGES as a hub and link various volumes, folders and network drives into this, making for a central places to do searches.
For those who care to pursue this further, here are some items I found. Junction point/ Reparse points definitely was a good start.
http://www.google.com/search?q=directory+junctions+NTFS
http://en.wikipedia.org/wiki/NTFS_junction_point
http://www.microsoft.com/downloads/details.aspx?FamilyID=9d467a69-57ff-4ae7-96ee-b18c4790cffd&displaylang=en
To get anywhere, it seemed essential to install the Windows Services for UNIX (SFU - free from MS; only the Client and the Auth are necessary to install). After some mocking around with settings and permissions, I got all drives exported, then tried my luck with "mounting" in various ways, but ended up only with "mapping" the drive. The SFU has a sufficiently detailed online help (look for mount/ umount). I then also installed NTFSLink from
http://www.elsdoerfer.info/ntfslink/?page=faq, to easily manage the links to be created.
A great article is here:
http://shell-shocked.org/article.php?id=284 (good explanation of the situation - with complete overview over all integrations)
On Namespaces and Dfs (Distributed Filesystem):
http://www.setup32.com/resource-guides/windows-2000-server/installation-and-configuration/basics/understanding-distributed-file-system.php
http://www.codeproject.com/w2k/junctionpoints.asp?df=100&forumid=75&exp=0&select=362535 also seemed worth looking at: "You could probably solve this by mapping a drive letter to a dir on a non-Micros~1 OS using SAMBA, and from some point in that tree use an NFS mount to... Well, you get the point. You'd need devilish fast net to make it work as-local though..."
All to no avail though, everything only seems to work for local drives, but not networked ones.
So the alternative solution of course is to have all image drives in one NAS Server, and search the root. This does not allow for inclusion of local directories in the path though. But through the remote storage option things remain scalable, albeit at the expense of performance probably.
Quite a waste of time this research was, without the desired result, but at least an answer found...