There seems to be 2 underlying issues behind this question

Fear of using selectors other than id/class

The usual rationale behind using a class/id rather than child/descendant selectors is because

• You want it to still work if you move the element to another location on the page
• Someone beat you with the don't use child/descendant selectors because they are slow stick (btw, they're not that slow, just don't go overboard)

The first point doesn't apply in this case because your elements are already being styled a specific way because of where they are on the page in relation to other elements. If you need to move the element, it is almost certain that it will need to have its styling adjusted as well in order for it to fit into your grid. There's no reason not to use child/descendant selectors in this case.

Unless you need to style each address independently, putting a unique class on each one isn't going to get you anything. Just use element/descendant selectors or give them the same class and be done.

Markup chosen purely because it makes the grid styles easier to work with

Choose good markup (group it logically, use descriptive classes/ids when you use them, etc), then worry about the styling.

More logical markup:

<footer class="primary">
    <ul class="addresses"><!-- could be a section instead -->
        <li><address>Somewhere
            1234 Maple St.
            Anytown, USA</address></li><!-- address tag may or may not be appropriate here -->
        <li><address>Somewhere Else
            5678 Elm St.
            Anytown, USA</address></li>
    </ul>

    <nav><!-- unless this is the primary navigation for the content this footer belongs to, nav is used here inappropriately -->
        <ul>
            <li>One</li>
            <li>Two</li>
            <li>Three</li>
        </ul>
    </nav>
</footer>

Sass:

footer.primary {
    @extend %grid;

    .addresses, .addresses li, nav {
        @extend %one-half;
        @extend %grid__item;
    }
}