If you want to create a client that is capable of mapping files to different FAT or NTFS drives, just export the drives of interest, and use the UNCs in your perforce client.

For example suppose I want to map a bunch of perl modules to d:/perl/lib/, gnumake and some other scripts to c:/local/bin, and a bunch of source to e:/. Using the win32 explorer, I share c:/local as //thelonious/local, d:/perl as //thelonious/perl, and e: as //thelonious/home. Now I can map gnumake and other script to c:/local/bin, various perl modules to d:/perl/lib/.../*.pm, and my current project to e:/project using the client below:

As a side note. I find it simpler to make a separate client for utilities and third party stuff than to include it in every project client. For both thirdparty and in-house utilities the client mappings tend to be non-trivial. For my home machine I want only the bare minimum I need to pull over a modem and when architecture matters I need win32. For my work machine I mostly want a full copy of things and typically need solaris versions of binaries. I find it preferable to get the mappings right once and be done with it, since these clients change rarely.

For builds, I create labels that are rooted at //depot, so I can add all the files from all the clients involved in a build, including the third-party/in-house-utilities client.