DON'T use a normal hoopskirt!!>-<
What makes a hoopskirt hold its shape, is the fact that the hoops are FORCED to stay as a circle, if you cut it open it will spring out and try to straighten. It won't keep its round shape.
This you could overcome by using something even more rigid, like a cage crinoline. Here's an example: http://www.sewingcentral.com/cgi-bin...t_id=6244_8419
I would recommend crafting it yourself, so you're absolutely sure it will hold its form and not collapse under the weight of the dress.^^ Plus you could make it short in the front and long in the back.^^
BUT there's more!!!! The weight of the dress you want to make will rest primarily on the back of the dress, and thus the whole contruction will swing forward (like a bell) and not sit as you want it to!! For this I recomend wearing a bustle-pillow (kinda like shown earlier) UNDERNEATH your crinoline, as this will prevent it from swinging forward.^^
I hope this made any sense, and good luck! I'm currently in the process of sewing a large dress myself, so thought I'd give you my two cents on the subject.XD