You might have better luck going the reverse direction - in Hype itself do not add a border radius, and then in your CSS make the class set a border radius on the element:
border-radius: 20px !important;
I didn't need to wrap in the conditionals; for some reason it seems the PIE.htc (library we use to add some IE effects) did not pick this up and try to put a border radius on the element. Definitely test to make sure this holds true for you. Also note that you may need the "!important" directive.