I just can't see a way to support IE8 and below.
Having said that, I don't want to sniff the user agent because it's bad practice and feature detection is a better alternative.
Would it be acceptable to wrap the entire content of my page in a conditional comment for IE9 and above therefore blocking IE legitimetelly? Or should I reconsider my plan?
This is entirely relative aka, it all depends. I've never had a case where I decided to give up on an IE version. Are you building with web standards? and testing? what's the major issue(s)?
A few thoughts:
No it is never acceptable to shut down browsers and shut out users. Never do that. Ok so you have to do it. Your plan to ice out < IE9 via cc is correct. Do that. Don't just serve up display:none. Give the user an explanation/context/link to newer versions, other browsers, etc.
Segue rant:
You are worried about best practices, so I have to point out that employing the best technical practices along with UX/UI practices is the way to go. Serving up nothing to a particular user is the worst thing you can do.
