Here is what I found in a quick search of the Biodiesel Forum. Use of Flax Seed Oil aka Linseed Oil, not recommended. It has a high IV (Iodine Value) and is classified as a drying oil. This drying characteristic is why it is used in paints and finishes. It tends to gum things up rather easily. I would suspect that these properties would be somewhat present in the processed Biodiesel. I would definitely recommend test batches.
Interesting. I seem to recall a number of years ago a custom harvester that pissed off another custom harvester in Colorado or Wyoming...and one poured a small bottle of iodine in the fuel tank of one of the other's combine...something about the iodine causing the engine on the $100K piece of farm machinery to seize.