This is something to try out to see if it makes for more interesting games.
Crouch (press c) while standing on or near to the assault rifle pickup just outside the base, and you'll be jumped up to the Double Damage ledge.
If you position yourself in exactly the right place you may just miss and rebound somewhere else (up or down depending on how you place yourself).
Sorry. You're not allowed to take spiders, rockets or flak up there.
It's currently running as a test. If it's fun then it may stay. Let us know how you get on.