layout.html

<link href="/stylesheets/screen.css" media="screen, projection" rel="stylesheet" type="text/css" />
<!--[if lt IE 9]>
  <link href="/stylesheets/pie.css" media="screen, projection" rel="stylesheet" type="text/css" />
<![endif]--> 

pie.css

// I want pie.css to only include IE-related styling:
.pie-element, #my-sweet-div-with-rounded-corners {
  behavior: url(/pie.htc);
}

// I don't want this in my pie.css:
#my-sweet-div-with-rounded-corners {
  -moz-border-radius: 20px;
  -webkit-border-radius: 20px;
  -o-border-radius: 20px;
  -ms-border-radius: 20px;
  -khtml-border-radius: 20px;
  border-radius: 20px;
}

pie.scss

// I would be awesome if there were some sort of silent import:
@import-silent "screen";

.pie-element {
  @include pie-element(relative);
}

screen.css

#my-sweet-div-with-rounded-corners {
  -moz-border-radius: 20px;
  -webkit-border-radius: 20px;
  -o-border-radius: 20px;
  -ms-border-radius: 20px;
  -khtml-border-radius: 20px;
  border-radius: 20px;
}

screen.scss

#my-sweet-div-with-rounded-corners {
  @include border-radius(20px);
  @extend pie-element;
}

Rather than trying to make the IE stylesheet apply on top of the base stylesheet, why not have it be a completely separate output stylesheet? Then you could do something like

.pie-element {
  @if $ie {@include pie-element(relative)}
}

One of your top-level stylesheets would set $ie: false and the other would set $ie: true.