Are there any workarounds for the CSS rule limits on IE?

One possible workaround is to split the Fooman Speedster output into several chunks.

Chunking is determined by resulting url length. You could adjust this by editing /app/code/community/Fooman/Speedster/Block/Page/Html/Head.php

find public function getChunkedItems($items, $prefix = '', $maxLen = 2000)
and change the 2000 to something smaller.

One thing to keep in mind while optimising performance is that using less rules is better.