I've scoured for months for a way to have Weakauras recognize the yellow text System Messages as it does for Raid, Whispers etc. without doing custom triggers. Has anyone had any success with this? Someone posted this same question a while back, but he said he found the answer but had a new unrelated question.
My solution has been to make a trigger for every possible chat type, and have it trigger on containing the letter "e" just to get a guage on what shows and what doesn't. This works for non yellow text chat some (e.g., BGs) but not all, for example anything involving qeueing.
I also tried editing Types.lua to include a System Messages to no avail.
chat_msg_system events should snag the bulk of messages but not everything that shows up in your chat window is the result of those events. You may be trying to fire on the the 'wrong' kind of event. In some cases it might make more sense to trigger on the events that cause text to be added (eg: battlefield_mgr_queue_* for queue BG-queue related triggers, etc) rather than trying to parse text in the chat windows.
Weak auras doesn't try to have drop-down/checkbox options for everything imaginable so it might not be possible to do it any other way. The custom functions are there for you to handle edge cases that would be so rare as to make the ui (even more) cluttered but otherwise be rarely used by most players.