i think I have a workaround that I've been testing today and has worked well so far on Mac and Windows 7 - file symbolic/hard links.
In Windows 7 -
C:\ProgramData\Hero Lab\data\pathfinder>
mklink /H "My Custom Userfile.user" "c:\Path\To\My Dropbox\Hero Lab Resources\My Custom Userfile.user"
Hardlink created for My Custom userfile.user <<===>> c:\Path\To\My Dropbox\Hero Lab Resources\My Custom Userfile.user
In Mac OS X Terminal -
mymacname
athfinder username$
ln '/Users/username/Dropbox/Hero Lab Resources/My Custom Userfile.user' './My Custom Userfile.user'
I haven't confirmed the Mac one but have been told by Mac/Linux users that would work perfectly fine. Will report back if it doesn't. Windows 7 however is tested and works perfectly, saving to the .user file in my pathfinder data folder triggered a dropbox update to my other PCs. Enjoy!
Update: confirmed working on Mac too! Updated the instructions