I've given it some thought and GitHub should be able to make this more accessible by doing something like this:

    <span class="mini-icon notifications"><span>Notifications</span></span>

Using media queries, serve CSS like this only to screen-like media (i.e. not screen readers):

    .mini-icon.create:before {
       content: "\f030";
    }
    .mini-icon span {
       display: none;
    }

Hopefully they'll adopt something like this, though it wouldn't help people like me who just want a consistent font across the Web.