Appending unique data to files before output

[Benjamin Jackson]

I'm working on a site which uses a quick digital watermark which is
added before download.
Ideally this might be added by nginx while transferring the files to
avoid tying up the mongrels.
It seems that http_addition_module does not allow for dynamically
added content (say from a query string match in the location). Any
idea if what I'm requesting is possible? Thanks,

Ben