In case anybody is still on this topic, I managed to get it to work 5min after my previous post, thanks to a suggestion on Processing forum. Using a virtual serial port emulator (http://www.addictivetips.com/windows-tips/vspe-download-free-virtual-serial-ports-emulator/) you can copy the stream from one port to another using a "splitter".
From my example above, I would take COM5 as input, create a COM2 copy, and write COM2 in the Processing sketch. I don't know why, but it works.
cheers !