Actually, Emulationstation has had some scripting added recently (see the docs about it), but it currently doesn't have any related events for screensaver start (the wake event might be used when stopping the screensaver).
Contributions to the project are always appreciated, so if you would like to support us with a donation you can do so here.
Hosting provided by Mythic-Beasts. See the Hosting Information page for more information.