Not me (yet - need discs first. One day...), but lots and lots of people are.
The most common fitment is a PBR VH44 (or sometimes VH40) fitted on the driver's side. I'm sure there'd be some photos around in the galleries, or someone else can post here.. Basically, you just mount it over/beside/behind the clutch.
I've heard rumours that there are actually three different pressures available in VH44's - they've been used in everything from Minis to old Holdens. Three pressures (different diaphragms), three different levels of assistance. I could be wrong, but it'd probably pay to source one from a Mini or Mini shop, although they are available elsewhere (could try Repco..?). There are also other types available, but I'd expect that they'd probably be a little dearer, as they tend to be replicas of the boosters that were originally put into S's (can't remember if that was standard, factory or dealer fit - someone else will know). Either one should do the job for you.
Hey, found a pic! 5th post down, 2nd photo, driver's side, little black booster mounted next to the engine in Peter1's very nice Cooper S. http://www.ausmini.com/forums/viewtopic ... c&start=15