>
> Compressed-filesystem extension (like e2compr, and I think either
> Fat or NTFS) can do that.
>
Windows (NT/2000/XP) can compress individual directories and files under
NTFS; new files in a compressed directory are compressed by default.
So if the 'spill-to-disk' all happened in its own specific directory, it
would be trivial to mark that directory for compression.
I don't know enough Linux/Unix to know if it has similar capabilities.