If by 'rated', you mean an officially approved breed show(?)-there is nothing in either rulebook(A or R)-that I know of-that would prevent you using the kind of EE cart you describe in a Country Pleasure class(it does have to have a basket, which I think is what you are referring to when you said "metal bottom". )However, it is a reality of the breed show ring that you *might* not be looked at as readily, all other things being equal, as someone, just as proficient and properly turned out as you, dress and harness-wise, who is using one of the 'accepted' types of show cart. Not saying that's the way it should be, just the way you might expect it to be. The 'everyday' type of metal EE carts would likely be more accepted at local, fun-type shows. Since showing SHOULD be about having a good time and doing your own and your horse's personal best, go ahead and show with what you have, as long as it fits any rule parameters and is safe and properly fitting-and have a good time!!
Margo