Possible legal issues with wetlands would make me think that someone would want to stick to small square bales since they shouldn't be as noticeable.
I believe there is a new pipeline coming through the area from NW MN down to Superior, WI. Rumor is that some people have been able to get some leftover mats for cheap once the pipeline work is done. They are best used for "temporary" access through swamp areas to avoid any legal issues. Here is an example of the mats they use:
Old pallets with some old hay spread on top may work. Some old hay bales ran through a baler buster would probably work as well, but hauling out to the swamp would take a while. Also hay is still pretty expensive, 10 years ago it would have been a fairly cheap project.
Also large equipment in the swamp would not be fun. If there is a contractor type guy that is willing to risk his equipment in a floating bog, even during the winter, I don't know if would trust him because he isn't very smart!
We have steel tracks that go over the tires on our skid steer and only go on the edges of some swamps when it is very dry in the summer and still get close to getting stuck. You can usually push out with whatever attachment you are using when it is dryer.
The weight of a large round bale with a skid steer would make you sink much quicker and give you a very good chance of breaking through during the winter when frozen. I am pretty sure most bogs do not freeze as deep. One of the skid steers with factory tracks would have a lot better flotation but I would let someone else do it first.
I did some searching to find a few pictures if have seen on the internet before.
Here are some snowmobile trail groomers:
An excavator that tried to a pull a groomer out, this was from the Merrifield area
http://brainerddispatch.com/news/2011-02-18/excavator-finally-pulled-swamp:
Looking at those pictures makes me say that I would stick to square bales that you can pull in with a 4 wheeler or snowmobile during the winter.