Perhaps this has changed in 5.6 but these were the steps I went through to enable CSS3 Pie pie.htc on the Author, Publish, and Dispatcher when I got it working in 5.4 and still works in 5.6.
The main problem I believe is that IE7 / 8 want the HTC Behavior to be sent with a very specific mime type of text/x-component
.
So in order to do that I'm sure there is some way to add a Mime type in Sling, but I couldn't make heads or tails of it at the time, so I just did something silly and turned pie.htc into a component that manually sets the mime type.
In /etc/designs/myapp/pie
I have an nt:unstructuered
node with: