I think maybe I didn't ask my question clearly. Here's what I want to do (very simplified example):

Key 1:
Machine 1: Attack target

Key 2:
Machine 2: Assist tank, attack target
Machine 3: Assist tank, attack target

Key 3:
Machine 4: Assist tank, debuff target

Key 4:
Machine 5: Target tank, cast heal

Key 5:
Machine 1: Attack off
Machine 2: Attack off
Machine 3: Attack off
Machine 4: Attack off
(delay a second)
Machine 2: Mez target
Machine 1: Taunt target

The idea is that I want keys to be able to send commands to different machines independently. I don't want to broadcast one key to all machines because the actions are asynchronous and event-dependent. I want to be able to have subsets of characters do different things based on one input key, if possible. I envision having about 20-25 keys set up for common sequences of stuff.

Look at it this way - the timings for taunts, debuffs, heals, nukes, and dots are very different, and I don't want to tie them together artificially.

In reading the forums it really sounds like the hardware and software solutions that are generally discussed either broadcast everything or have a choice between sending to the master machine or broadcasting. I did read quite a lot over the course of a couple of weeks before asking my question - hopefully I didn't miss something obvious.