I am pretty sure that this is not possible. You might be able to transfer the file via ftp or a mapped netowrk drive, but then the user would have to open it manually.
Even with embedded vis in a remote panel, you are just controlling things, and all the files are local on the server or linked to a mapped drive.
You MIGHT be able to accomplish this if you had a client vi running on your machine, that would get the file from the server, and then run some activeX commands to open the spreadsheet. and then after you save the file, have the vi send the file back to the server. but this would require each user to have a client vi or exe running on their computer. Would be a cool project to try, but would take some time to implement.
Kenny