@jstrout I spent a day on this last week, here are my findings and where I left off:
The official editor ("ANTLRWorks") seems to be for the older version of ANTLR (ANTLR 3). ANTLR is now on version 4. I tried editor support for version 4 in 2 ways:
Using IntelliJ and the official ANTLR plugin. This was more confusing than option 2, as I could not figure out how the files were supposed to be arranged to get into a REPL flow. 
Using VSCode and an ANTLR plugin written by Mike Lischke. I eventually figured out how to be in a REPL flow here. 
The problem is... I have no idea what I'm doing or what this language is... I figured I need to understand the language by looking at an example for a language I know well, such as this ruby-like language which has an ANTLR 4 example here. I felt overwhelmed and moved on to other responsibilities and have not tried to visit again until today to produce these notes links and images.
It just seems like too much time investment into an esoteric language and toolset simply to get syntax highlighting (my only reason for doing this at this time, I am ignorant of other grammar magic at this time) for me to push through the learning curve I started to hit. This is especially the case when I can get syntax highlighting for ACE using the mode you already created, but it just means I'm stuck using ACE I guess... But then it's probably easier to understand and convert ACE to Codemirror or whatever other editor, than to grok ANTLR!
Hope that helps get the ball rolling, I'd love for arcnor to tell me something that makes me want to learn ANTLR given the above argument with respect to syntax coloring... i.e. what other magic makes ANTLR worth learning?
