I know this is in development, but may I make a suggestion for when you do get it moving in the future...
See the attached picture, it is from a reporting system I use, but it has the right type of interface for what we should be able to do. where each line is a trigger or condition, each line can be selected to be in a bracket you can hit "and" or "or" between lines, and outside of the captured area you can click to add more lines. It cant go as complex a PLEG, but would be fairly intuitive....especially if when you hit "validate" that it will stop you if you dont have a closing bracket for one that is open.