The only way I could see getting this data into lansweeper would be to create a start up script that would check the size of the my documents folder and write the size to a registry key. Then capture that key with custom registry scans.
Note: This isn't the best idea I've ever had since it will calculate the my documents size every time someone logs on and by your own admission those sizes could be large and therefor take a log time, but it will work.
http://blogs.technet.com/b/heyscriptingguy/archive/2005/03/31/how-can-i-determine-the-size-of-the-my...Const MY_DOCUMENTS = &H5&
Set objFSO = CreateObject("Scripting.FileSystemObject")
Set objShell = CreateObject("Shell.Application")
Set objFolder = objShell.Namespace(MY_DOCUMENTS)
Set objFolderItem = objFolder.Self
strPath = objFolderItem.Path
Set objFolder = objFSO.GetFolder(strPath)
objShell.RegWrite "HKLM\SOFTWARE\Information\MyDocuments\FolderSize", objFolder.Size, "REG_SZ"
Modify last line of code to a registry key you want and set lansweeper to scan it.
-Sal