I have another idea about GUI. I didn't knew about shepherd before, so my girlfriend and I developed another application of this type. We started from the other side. You developed action and trigger framework and now think about language, that would tie them together. We have taken SWI-Prolog which gives very handy logic expressions and power of true complete programming language and added event-action framework and temporal logic operators. So we have simple tools for expressing things like "Do smth. if A started to happen and within last 30 minutes B was always true and existed such moment, that C was greater, than 50".

It occurred, that good documentation is needed to make people understand how to use it. Right now we have not much time because of graduation, but we try to improve it from time to time.

So, IMHO, GUI shouldn't express all that tool can give you. Look how many daemons with GUI there are for n900. It would be much easier to develop only GUI, which would write scripts for such language. There also could be GUI for constructing finite automates: create node, describe what conditions should have device in this node, connect nodes. If conditions are not met, switch to connected node, for which conditions are met. Describe what actions should be taken on transitions.
But I think, that GUI must be separate package from the daemon, which runs scripts. And I also think, that from theoretical point of view our approach with complete programming language (we called it ET-Prolog) and temporal logic has much better descriptive capabilities. But at current state it seems, that shepherd is more usable.