I use a lot of bean transplants for some of the same reasons you
mention (small seed samples, long DTM, higher dry seed production, etc.) but plan to put them in the ground at the same time as beans
which will be direct seeded. If good conditions are late in coming, I will usually hold the plants in a protected location until the weather and/or soil temps improve.

Bean seedlings can tolerate low temps for brief periods (anything above freezing) but will likely die - or be weakened - if those low temperatures continue.

In my experience, bean plants put out too early are often stunted, and those that are direct seeded later (under warmer conditions) generally catch up with them. Not to say that it isn't worth trying... but have a back-up plan, such as starting transplants in waves. If seed is limited, it is best to start transplants later (such as 2 weeks earlier than you expect to direct seed) and to hold off transplanting until warmer conditions arrive.

Not sure how beans would do in a hoop house. I once tried growing bush beans under cover, to get pure seed; I used the lightest grade of Agribon, over a PVC cage. The beans in the cages were sickly, with yellowed & spotted foliage. When I removed the cage, the new foliage was green, and the plants returned to good health. They might do better under a cover that allowed better air circulation (such as the mosquito netting Rodger used several years ago) but with better air flow, you would lose the benefit of heat retention. Unfortunately, any cover that will trap heat will also trap moisture, and beans are susceptible to disease under moist conditions.

They need about 65F soil temperature for germination but they can do well with cooler air temperatures once they emerge. Here south of Seattle we see nighttime temperatures as low as 40F even in mid summer and that does not seem to cause problems. Perhaps that is too cold for some varieties but many Appalachian heirlooms do fine here.