It should just work, if you use the latest v2.0 product (v18.104.22.1688 or later).
For a long time the syntax where you negate the RewriteCond with a leading ! was not supported in IIRF. you had to use a (?!pattern), which was a pain.
As of IIRF v22.214.171.1248, the leading ! in a pattern on a RewriteCond is treated as negation. The rest of that file is fully compatible with IIRF.
So... use v2.0 (latest) and you should be good to go.