If sponge is a forge core mod, and forge has a client version, even if you do not need forge or sponge on your client to connect to a Forge/sponge server, then wouldn’t putting “Sponge.jar” or whatever in your client’s “mods” folder have the same effect on the local “hidden” server as it does/would on an actual server, allowing one to use sponge plugins, such as say, sign edit, on their client?
(Disclaimer ~ This is just my basic understanding without reading any code)