Write our own WebSocket server transport plugin
The flash proxy README currently documents how to run the websockify (https://github.com/kanaka/websockify/) program as a ServerTransportPlugin. This works, and it does base64-encoded text frames that allow sending binary data. However, the program is rather larger than we need it to be, and hasn't been audited for security as far as I know. It also has an unwanted built-in web server (that serves local files). The built-in server is disabled by default, but easily enabled.
Solve these problems by writing our own, with an eye towards easy deployment and ability to work as a managed proxy.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information