I'd recommend summersweet clethra. It comes in white or pink, grows to about 5 or 6 feet and has fragrant flowers from about mid July to mid August. This plant has a very clean appearance even without pruning

Coccoloba uvifera - "Sea Grape" .... Is readily available, and can be easily grown from seeds. This tree is sold commercially, retail and wholesale, and can be trained as a single-stem, or, in bush form. When the stems are thin, they can be bent and twisted together, or you can try approach grafting techniques, depending on your pleaching pattern. It is a very tough plant, will offer privacy, and the evergreen leaves are usually disease resistant. It is a native to tropical climates.

Janey, I don't know of any silvery Aussie natives that might be acceptable to pleaching, however, the plant that comes to my mind that might be worth a try is a variegated pittosporum. It has leaves similar to Acmena or Syzygium (excuse spelling), which grows to about the size your looking for, 2.5 to 3 metres. I haven't grown it for some years but it should still be available from large nurseries.

Do all the plants need to be salt tolerant or only some? Does the tolerance include foliage and airborne spray, or roots and salty runoff, or both? Plants don’t make much of a difference to sound levels; only something solid like a wall or solid fence can do that. Plants can provide visual screening, however, and if planted on a berm, the berm can provide some sound blocking or reflection. Most of what grows that is native in zone 4 isn’t evergreen except for conifers, and not many of them are salt tolerant IME. You could also try growing from cuttings if other folks you know would let you, although it wouldn’t have genetic diversity unless you took cuttings from different shrubs and trees within species. Just so you know, Rosa rugosa isn’t native or evergreen, though it has naturalized over a fair amount of the Northeast. There are native roses such as R. palustris AKA swamp rose and R. virginiana. For evergreens, balsam fir as it grows on my property is relatively narrow, but as with most of these plants, I don’t know about salt tolerance. Check out junipers as another hardy needled evergreen that may have some salt tolerance. I can grow Rhododendron maximum, but it does get winter foliage damage in sun. My mountain laurel AKA Kalmia latifolia Seem to do better with keeping their winter foliage looking nice, but are mostly in at least part shade, so that might be the explanation. I don’t think either is salt tolerant. Yew tend to be eaten by deer if they are an issue for your site, and often Thuja are eaten into lollipops as well, with all foliage missing as high as the deer can reach, so you may want to set up protection for them this fall, either netting on posts or a webbing of posts and high test fishing line which they have difficulty seeing. For deciduous plants, look into the native Viburnums, some of which should be hardy enough for you, but I don’t know about salt tolerance. Physocarpus AKA ninebark isn’t evergreen, but it is densely twiggy and so provides fairly good visual screening and is a US native. Hydrangea arborescens is native, but in my garden doesn’t get taller than 4’-5’ and prefers shade in the afternoon. I have seen Amelanchier AKA shadblow or serviceberry growing in the median strip of the interstate, so it must be at least slightly salt tolerant. There are several deciduous dogwood shrubs and Ilex verticillata AKA winterberry holly that have winter interest. If you are looking for an inexpensive source of seedlings so you don’t have to grow your own, check out the NH state Nursery. Order early next season (check in January) since I think that they may be done shipping for this year and they often sell out of popular plants. Many, though not all, are native plants.