I will check how to do the jquery but it might be dificult as onion should be compilable on non debian as well. Is is ok if at compilation time it is decided to use the onion provided one, or the system one? If the system one is used then its is not compiled in (jquery is converted to a C file to be used as static data).
About systemd, onion will never force the user to use it. The support is just to compile the functionallity to be able to cooperate better with systemd, if the developer decides so. It is always optional to use systemd, but the library must be prepared. Even if the developer of whatever uses onion decides to support systemd, it still can be used without it.
http://0pointer.de/blog/projects/socket-activation.html