Hang 3 flat-screen monitors from a tilted wooden beam
Is mounting a triangle shaped hardwood block to reduce or eliminate the 30 degree angle out of the question? I would lag-bolt it to the beam and then mount the ceiling mount to that in the usual manner.

There are plenty of long extension monitor/TV ceiling mounts available which with the appropriate length, put the monitors at eye-level, but as you've found, 20 degrees is as much as the ceiling end will compensate for.

Also, several of the multi-monitor mounts use a chrome tube and the brackets are movable up and down the tube by a thumb screw arrangement. Nothing prevents you from changing the chrome tube out for your own custom length one that would extend from a drilled hole in the beam down to a floor or desk mounting block. 

Having it supported on both ends also means you're not sporting a really expensive pendulum that bounces around in air currents or from the garbage truck rolling down the street.