I have some experience making posters for conferences, so I will share in case anyone is up for stepping up.
It is visual, straightforward, and a good case for Lilypond.
Once you have grabbed someone's attention, you may include one or two side stories. This must be short and sweet, but feel free to go into technical details. For example, the ability to engrave Gregorian chant, the possibility of arbitrarily expand it with Scheme code, or that you can write the notes in any language (yay! I don't have to spend time thinking what note D is!).
The poster shouldn't necessarily stand on its own if someone is willing to be there during the poster session to explain and expand. You definitely don't want to put a cheat sheet or a manual because no one is going to learn Lilypond at the poster session, the goal is to make it memorable so they go home and learn it. Snippets are fine, but completeness is not required nor desirable. A personal workflow can be useful on the side, for someone already familiar with Lilypond or convinced of its coolness.
Re-reading my email I realised I may sound a bit too harsh, so please do not take it badly, it wasn't meant like that. I have been to plenty of conferences with professional scientists, that are supposed to do this for a living, with terrible terrible posters. The result is that people just gloss over, and only the two people that were already interested (and likely knew the project) ever talk about it. Also, in an engineering students recruiting fair, I saw plenty of posters that seemed aimed at investors, so recruiters seem to be bad at this too. There are very few good examples to learn from in the wild.
I hope this helps.