It's all open source. Here's HHVM: https://github.com/facebook/hhvm. What's stopping you from working on it now? For all intents and purposes you would be hacking on it with them.
I am, hence my parenthetical. Currently on the application/tools side, building some infrastructure and pushing the language to find it's rough edges, but I've been learning OCaml to be able to get stuck into the code directly.
On the other hand, there's something to be said for working directly with the main developers in a room with them!
On the other hand, there's something to be said for working directly with the main developers in a room with them!