There is a solution using symbolic links managed by the filesystem.
My friend Ben and I worked out how to do it this morning so that we can use Dropbox for the .user files exactly as you want.
Anyone who has used Linux or Unix would be familiar with symbolic links - they create a "symbolic" file at a location which is actually a mirror of a file in another location. You can do it in Vista and Windows 7 too. Check the wiki here on them:
http://en.wikipedia.org/wiki/Symboli..._symbolic_link
So to get this working follow the following steps, this will make HeroLab think the .user file is in the Program directory, but it is actually just a link to the real file in your Dropbox folder.
1) make sure your Dropbox folder is setup for auto syncing the .user files etc on the computer you have HeroLab installed on.
2) type "cmd" into the search bar in the start menu.
3) in the terminal that comes up navigate (use the 'cd' dos command) to your HeroLab folder which normally contains the .user files. i.e. type "cd C:\ProgramData\Hero Lab\data\pathfinder"
4) use the 'mklink' dos command to create the symbolic link as follows (change the folder/file names as appropriate):
mklink /H "Pathfinder-Forgotten Realms.user" "c:\Downloads\My Dropbox\D&D\Hero Lab Resources\Pathfinder-Forgotten Realms.user"
(i.e. mklink /H "userfilename.user" "\<dropbox folder>\userfilename.user")
This should create a symbolic link file in the program directory which links to the real file in your Dropbox, which will keep up to date and synced across all your computers.
Ask if you have queries.