buckwheat is my preferred cover for tilling in. soybeans are good for choking out weeds such as wiregrass. i do not recommend clover of ay sort.

while i'm not in the beautiful willamette valley, i did live there at one point.

I planted clover in two of my beds last fall. Around March 1st, it really took off. In April, I cut the abundant foliage and put it in the compost. During a brief break in the rain, I tilled the stems and root system in. What did it do? Not sure, but the garden looks great!

I have used crimson clover. It needs to be established before frost because it will not grow much in the winter time but grows vigourously in the spring. Crimson clover is kind of hard to till in because of the tough thick stalks it has. It has a good root system that is beneficial to green manure also.

For legumes, you might consider black eyed peas or fava beans, which can be bought cheaply in one pound bags at the grocery store.

Jim

You could try some fenugreek. You can buy a pound of spice seed from Penzeys or any Indian spice store and it sprouts easily. It's a cool season legume, plus it will attract beneficials. Annual clover of fava beans , English peas are other good cool season ideas, too.
You could also mix a grass like annual rye with the legume to add more organic matter to the soil.