I think you're probably outta luck, unfortunately.
There's no documented way to assume the identity of another user in Windows w/o using their password. That "problem" underlies what you want to do. There are definitely times when I wish I could just "su" to another user's identity...
It sounds like you want to replicate the specific actions taken by Winlogon and Userinit after the user authenticates to get the local profile "established", and then populate that profile directory with whatever contents you want.
The detailed steps taken by the OS during logon aren't documented anywhere that I'm familiar with at such a level of specificity as to make that possible. (Wouldn't it be nice to be able to take a peek at the Windows source code at times like this?). If were to try and do it, as a "hack", I'd start looking around at the registry where Windows keeps the "profile list".
The "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList" sub-key for the user gets loaded with information about their profile. It would be tempting to attempt to pre-create this subkey for a scratch-monkey user and see if you can get Windows to "recognize" them as having a valid local profile. You'll need to render their SID as a binary blob for the "Sid" value in that sub-key. Raymond Chen can tell you how to do that here. Many of the other values in the sub-key are self explanatory, but some aren't. After just fighting last week with Vista refusing to load a roaming user's profile and finding no documention to help with the problem (but determining that the problem had something to do with the "NextLogonCacheable" and "State" values in the sub-key for the profile), I can tell you that the documentation from Microsoft on those other values is pretty weak.
All-in-all, I'd say that you're probably on the business-end of what's going to turn out to be more of a "person problem" than a "tech problem", unfortunately. (I know those tech-challenged users only too well... Somehow I still have hair, even for want of pulling it out so many times.)