I agree that roses in the Canadian series - roses bred by Svejda, Colicutt, Marshall, etc. - are a great place to start when looking for cold hardy roses. That said, I'm not sure it's a great place to start looking for cascading roses.
I started out intending to flesh out Zuzu's idea of using rambling roses. Rosa wichurana, the species rose used to produce them, was really a good groundcover rose, but I ran into certain difficulties. I grew Super Dorothy in NJ and in five years its whiplike canes never got two inches off the ground. Sadly, it hardly grew at all, so I was not impressed with that cultivar. Here in Arizona I am growing Francoise Juranville. In late spring I ripped it out of the ground, buried it unceremoniously in another part of the garden and forgot it for a while. Two weeks later I remembered it. All the canes looked dead. Then I started to water it. Now it has several long, healthy canes. This makes it one of the more vigorous roses in my garden. It's whiplike canes could probably be trained to follow the landscape in precisely the way you wish, and its warm pink flowers are irresistible. It can suffer from mildew.
In search of good ramblers for this project I went to HelpMeFind and chose ramblers hardy to zone 6a and resistant to disease. This search produced only the setigera rambler described as "Moser House Shed Rose." Sadly, this rose is not in commerce. So I guess one would have to be ready to treat for mildew if the rambler idea were pursued.
There are a number of floribundas or hybrid musks that might possibly be worthy of consideration. I can remember seeing Iceberg used as a ground cover. Many rose gardeners consider Iceberg to be overused; but it still looks novel to people who don't grow roses. Similarly, The Fairy might have characteristics not too far afield from what you seek. If you seek dark red, Europeana might be a possibility.
I would like to suggest that there are a number of roses sold as ground cover roses that are bred for precisely this purpose and might work well. The first that comes to mind is Sea Foam, a tough, hardy rose that produces white flowers. Not sure whether it cascades enough; only boulders larger than about three or four feet high will really penetrate its facade. It produces flowers in abundance, and it has neat dark green glossy foliage complements them nicely and is dense enough to shade out weeds. I mention it because it provides a benchmark against which to measure roses in this category.
There are a number of other good ground-cover roses that are old and well proven, roses that are cascading, foliferous, and cold hardy: Red Ribbons, Immensee, White Flower Carpet, Surrey, Sussex, Kent are among these.
This year I purchased from
Palatine Roses in Canada a rose that clearly has inherited some of the same wichurana characteristics that makes Sea Foam a good rose. Like Sea Foam it has dark foliage and almost unconditional vigor. It bears watermelon pink flowers in great profusion and over a long period of time. It's called Toscana Vigorosa and it's one of a series of Vigorosa roses introduced by Kordes. (Kordes' grandfather crossed Rosa rugosa with Rosa wichurana to produce Max Graf, a vigorous and foliferous ground-cover rose that essentially created the category. My guess is that many of the roses in the Vigorosa series make special use of that cross. In any case, Kordes has a long history of producing extraordinarily tough, cold-hardy roses that should be well suited to coastal Maine.) I would recommend Toscana Vigorosa without reservation if it were not that the flowers fade quickly and I don't like the look of the plant at this point.
Kordes' history of producing tough roses along with my own experience with Toscana Vigorosa suggests to me that roses in the Vigorosa series could be ideal for your project. Finally, breeders Poulsen, de Ruiter, and Olesen have done a lot of work in this area, too. One can sift through their work at HelpMeFind.com.
I know there are a few very different ideas in here: I hope you find some of this to be helpful.
