This '04 JCW has an engine noise I'm trying to isolate from the top end or the supercharger. How hard is it to remove the drive belt?

You only need a belt tensioner tool

Check eBay or any Mini Shop.

Just remember, all your accersories will be disconnected....including your waterpump...so do your checks fast...
You can use a socket as a fulcrum, and a pry bar to compress the tenssioner...the have a buddy put a pin in the hole on the tennsioner to lock it...then simplely remove the belt....a 2 person job...but it can be done sans the tool.