I know about using the dual Cat5 cables, but I just stumbled across an old round D-sub 25 parallel 10' cable that used to run a laser printer. 25 tiny discrete wires already broken out to gridded pins in a very flexible cable a bit smaller than the average pencil.
Also, my cable is old enough that the connector covers aren't molded on, they are snap-on.
Then I remembered that there were also DS 15 cables, which last I saw were being used for DVI-D monitors & such and were up to 15' (4.5 meters) long.
These are so obsolete that they may have been trashed already, but if you can find'em they should be seriously cheap.
So... before I cut mine up, is there any obvious reason these cables wouldn't work out as Pi extension cords?