Sounds like you can build a System with a reference now, according to Ralith, so it should be possible to do rendering with it as well. Check it out!
WRT rendering, at the bottom of their guide to using the new thread-local systems for this, they mention they are looking for PRs with examples of usage from other frameworks.
A very simple example of ggez+specs - https://github.com/TatriX/ggez-specs-example/blob/master/src/main.rs
My newest project is built off of ggez & specs! No idea if anything I'm doing is "best practice" but at least it's another example. https://github.com/obsoke/arpg
Awesomeeeeeee.
Actually https://github.com/ggez/game-template is kinda turning into this, so that might just do the job.
Yeah I think https://github.com/ggez/game-template is going to be the canonical example for "how to integrate with other stuff".
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A very simple example of ggez+specs - https://github.com/TatriX/ggez-specs-example/blob/master/src/main.rs