I have never grown flax but my understanding is that it should be grown as you would grow spring oats. Plant in the early spring.

Yes that is correct.

Any kind of grass or grain or herb seed, can be used as a cover crop, green manure crop, or even a companion plant.

Flax is a wonderful plant. We did the same - had lots of food flax from the health food store and thought we'd try planting some. Now every year when the weather has warmed we plant a row of it alongside the potatoes, for which it is said to be a good companion. Very pretty flowers. Doesn't self-seed so it hasn't become a pest. I would harvest the seeds if we had some kind of equipment for separating out the chaff.

I don't see why you couldn't use flax as a green manure. I don't recall ever having trouble with the dry stalks persisting so they must break down okay (we leave them in over winter; we till the garden only once, in spring). Flax isn't very leafy though so it won't contribute as much organic matter as clovers or buckwheat for instance.

Mine is organic seed from the health food store too.
About how much would I need to plant for a 100 square foot bed?

Take a look at what the Thomas Jefferson Institute has to say about Flax. They talk about spreading 50 pounds pre acre of seed.

Here is a link that might be useful: Thomas Jefferson Institute

If you grind it up flax seed should be an excellent fertilizer.

Here's a link to a good conversion site - eg. acres into square feet. I don't measure; I just sow the flax thickly - maybe four seeds per square inch, which is overkill, but the seed is cheap and I want lots of it growing, and the earwigs or slugs always mow down a lot of them - in a band 5-10 inches wide, then rake in and water.

Here is a link that might be useful: convert just about anything

That's a great link Paul, thanks for sharing.

I've done a few differentt green manures with an assortment of plants. All of them, IMHO, break-down faster in the soil if you turn them in just as they're setting bud. Once they set bud they start to get a little woody to support the weight of the stems and flowers, then seeds. It's just a LOT easier to turn the plants in while they're still all green.

Trudi what a surprise to see you. I just looked up zeppole btw. Hmmm.

I did not know flax could be planted earlier; now that I do I will, as early planting might help the flax escape being eaten by pests.