We are using Phabricator in a software company develops embeded linux software , protocol servers and sop on. Comparable with tzhe LLVM project using phabicator as well. We like to have some sketching feature like the pholio mockups. However we are more about diagrams, message exchanges, state diagrams, flows and sequences. as we like textual languages we are using plantUML for years to create the diagrams.
It should be possible to include plantUML syntax as a meme or image macro in text areas. The picture or svg is rendered by using either a locally installed plantuml.jar or by using the plantuml server in a similar way of using elastic search.
for reference:
* http://plantuml.sourceforge.net
* http://plantuml.sourceforge.net/running.html