I'm not in Maine so I don't know enough about your climate to comment on anything related to that, plus different pumpkin varieties grow differently. I also don't know you native grass or how it grows. Bermuda grass is my biggest problem here.
I anticipate two problems for you. Those pumpkins are likely to run a long way. That's where variety comes in, some can go a long way. They won't stay on your covered and mulched area. They will be running all through the grass and you won't be able to mow the grass or keep the weeds down. If you step on or crush a pumpkin vine, you can kill that vine from that point out. That's typical for many varieties of pumpkins, melons, and vining squash. You will probably have a weedy grassy jungle where those vines run. It's not the end of the world, lots of pumpkins grow like that. The big problem is that it will not be very attractive, which may be a factor if you are thinking of a pick-your-own operation. You might consider expanding your mulched area out tot he side. It doesn't have to be that ground cloth, I use newspaper or cardboard boxes and cover that with mulch. Then try to redirect some of the runners onto that mulched area.
The other problem you might have is that the grass and weeds will grow through the cross you cut to plant the pumpkin. That's going to compete with the pumpkin for nutrients and maybe sunlight until the pumpkin starts to run. The only thing I can suggest this year is to mulch around the pumpkin as tightly as you can and try to keep the stuff cut back and pulled out as well as you can without damaging your pumpkin plant. It will take some preplanning, but next year I suggest you dig out an area at least a couple of feet diameter where each pumpkin plant will go and sift through that to remove any roots, especially grass roots, that you can. Any weeds and grass stems need to come out too. Then make some compost using your chicken manure and mix in some compost where you are planting a pumpkin. Hopefully you already have a nice compost pile going. If you stick with us, we'll have you composting if you are not now. Clearing an area away from the opening will slow the encroachment of the grass and make it easier to pull anything that pokes its head up without damaging your pumpkin plant roots.
Overall I like your plan. As you mentioned there is a learning curve. Maybe it will work out to your satisfaction without doing either of these. Good luck and let us know how it goes.
Note: You might modify your profile to give us an idea of where you live. It does help in answering a lot of questions and in understanding what you are telling us.
