We don't have a "permanent" preventer -- we've already added an abundance of control lines to our Mk I so we don't want to clutter it further  (see
www.catalina36.org/comment/55992#comment-55992) -- but we do have a system that can put into operation rather quickly whenever it looks as if we're going to be heading downwind for any appreciable period of time.  

     It starts with a "permanent" extra pair of Garhauer track car single blocks (http://garhauermarine.com/catalog_process.cfm?cid=28) , one on each outboard genoa track, roughly midway between the 2d and 3d stanchions from the bow (not counting the pulpit).   We have two preventer lines kept in the cockpit locker ready to go with snap shackles that we can attach to the aftmost bail on the boom (never attached directly to the boom; I've read that if something goes wrong a bent boom is a possibility, better to have the bail fail).  The respective lines then run through the track car blocks and thence back towards the primary cockpit winch on each side.   The assumption is that if we're running wing-on-wing, the opposite primary winch is available to take the preventer line (and can easily tighten or ease it, and quickly release if needed).  

     Photo below

Ed
I have a Dutchmen boom brake installed on my MK 11. So far so good. It's brake system that I set from the cockpit. Anchor points are aft shrouds.
I see your are in Ventura, so am I. Come by and take a look if you want. I'm headed out to Catalina for a couple weeks.
Chuck