so you need to look at what it says. These are normally drafted very precisely, and with lots of those words that they told us NOT to use when we were in law school, but which we all use anyway--just because it's fun to befuddle the natives, I guess. If it says that it must be "cedar shakes" (or some like language), then using a substitute may put you in the position of doing it twice if you get caught. Given that you are already fighting with the Association, you can best believe "they" (as in all your neighbors) will be watching very closely to see what goes up on that roof.
If you want to have a substitute material, I strongly recommend that you have a rep from the company who sells the material come to a meeting of your Association and do a demonstration. This would typically include showing both the real thing and the substitute together to show how closely they resemble each other, and then doing a presentation on the relative benefits of each type of material. Who knows, you could get lucky and discover that a number of people in your subdivision are looking for a no-maintenance version of their quaint rooftops. If you can get the majority on your side, you may be able to get the covenant changed to include the substitute material. You could also try a petition drive, where you go door-to-door and talk to your neighbors, taking with you a piece of the real thing and a piece of the fake stuff, along with some good photos of a completed job. Then try to get them to sign a petition saying that they think the substitute should be permitted. This is a lot of work, and you need to be prepared for a not-so-warm welcome at some (most/all???) homes.
Architectural and aesthetic covenants are absolutely enforceable by the Association, and in most cases, by any individual member of the Association (where no variance has been provided by the Association) (what this means is that unless you get approval, not only could you be sued by the Association, but each and every individual neighbor may have the right to sue you, depending on the language in your covenant agreement). I do NOT recommend taking your chances on getting away with it without their approval.