Trooper, my knowledge of plants pretty much stops at the water's edge. Meaning that up until recently, all the plants I have dealt with are were land plants. But, I can say that transplanting vegetation can be hard on them. In your scenario, your supplier will be uprooting the pickerlweeds to send them to you, then you would need to plant them with soils and water in the tub, THEN transplant them again into your pond. That's a lot of roots being disturbed. I do not know the pickerelweed to the extent of knowing how easily they are to transplant. I'm just saying that you will be reducing the success rate to some degree. Maybe not a big deal, just my 2 cents.

I would think that a standard plastic tub/bucket would be safe for your waters. You would want to allow the pond water to freely flow in and out of the tub to avoid temperature problems and stagnant soils and I would think the tub would need a lot of holes in it to allow the soils to breath as well. You can buy aquatic plant baskets and I have heard of people using burlab cloth for containing root systems of water veggies.

I do know that pickerelweed reproduces by seed so you will not be stopping that cycle unless you plan to keep your "tub o' weeds" away from the pond during the time the seeds are mature.

I'll be keeping an eye on your post as these plants are on my list for my pond.

Fish on!,
Noel