I'm surprised no one mentioned passing it in "additionalViewData" and reading it on the other side.
View (with line breaks, for clarity):
<%= Html.EditorFor(c => c.propertyname, new
{
htmlAttributes = new
{
@class = "myClass"
}
}
)%>
Editor template:
<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<string>" %>
<%= Html.TextBox("", Model, ViewData["htmlAttributes"])) %>