As EJS seems open source, I would propose to check if [url=][/url] could fit your requirements. It is public Free encyclopedia that allows user-contributed applets after the source code passes public code review. Applets are compiled on server side and all signatures stripped down, so they always run with minimal rights of unsigned applet without access to the local filesystem. The server also adjusts applet tags depending on the browser.

The site build system allows to specify "default libraries" that need not be reviewed nor downloaded separately with every applet. Maybe EJS could be one of them.