I use the tmux from MacPorts, not the one from iTerm 2. With that tmux, hit the prefix key then page-up (fn-up arrow): control+b fn+up

This applescript should clear scrollback for all sessions in the current iTerm window:

tell application "iTerm"
    tell current terminal
        activate
        set myCurrentSession to the current session
        repeat with theSession in sessions
            tell theSession
                select
                tell application "System Events" to tell process "iTerm"
                    click menu item "Clear Buffer" of menu 1 of menu bar item "Edit" of menu bar 1
                end tell
            end tell
        end repeat
        select myCurrentSession
    end tell
end tell

To bind that to a keyboard shortcut:

1. Open Automator and create a new Service.

2. Set "Service Receives" to "no input" and select "iTerm.app" as the application.

3. Put in a single Run Applescript action, and paste in the code from above.

4. Save it as clear-all-scrollback-buffers-in-current-iterm-window.

   Now when iTerm is open, you'll see that service in the menubar under iTerm > Services.

5. Open System Preferences > Keyboard > Shortcuts > Services. Scroll down until you see clear-all-scrollback-buffers-in-current-iterm-window. Give it a keyboard shortcut (eg, optioncommandk)

If that doesn't work, you might try the following AppleScript instead:

tell application "iTerm"
    tell current terminal
        activate
        repeat with theSession in sessions
            tell theSession
                select
                tell application "System Events"
                    delay 0.1
                    keystroke "k" using {command down}
                end tell
            end tell
        end repeat
    end tell
end tell