Excel cache appliance
 

I want to exploit a cache appliance for xls files. There are a plethora
of "backup copy" plug-ins, but I want to catch file open and close to
write through a cache to the file system. One option seems to be to
create different file type (eg, XLF). Bytes would be idential to XLS.
Then, separate read/write, like for csv.
Are there other approaches? Are the interfaces to build an Excel
reader/writer documented?