Hi,
There is no event that you can get notified when document saving performed over. If you want to do it that way, you need adopt timer and some synchronizing logical.
In fact, I think we can try to avoid the error message you said that popped up after saving the file to another place. Excel 2003 Document embedded the manifest in the file itself. The manifest points to where customization assembly locates. With manifest in itself, VSTO solution loader knows where and how to load corresponding assembly codes when the document is opened.
You will receive that error after save the file to another location because, by default, the assembly location use relative path in embedded manifest. This makes Excel only tries to look for the assembly in the same location it exist. All we need to do is modify the assembly location using full path format.
There is a tool can help you manage it. See this article:
http://msdn2.microsoft.com/en-us/library/aa537185(office.11).aspx
Thanks,
Ji