I'm not quite sure I understand your issue here. If you require a password on screensaver, it will not stop processes from running or block you from remotely accessing your Mac. I have this option set (using ControlPlane to activate it at work and deactivate it at home) and I have never once had the system refuse to respond to a Remote Desktop, VNC, or SSH connection. In all cases where GUI access is required I just need to input the password; SSH uses key pair so no password is requested. If you don't want apps to sleep, then make sure the system power settings are such that it will not enter system sleep before you return. Just set the sleep timeout to a long delay. If you just don't want to be bothered to enter a password when you are sitting in front of the computer and working, then increase the screensaver delay to a longer time before it activates, or add a 1 min delay between screensaver and password prompt so that you have a chance to catch it before it locks you out.
I use ctrl-shift-eject to lock the screen. It's fast, simple, and it works. This is the built-in way to lock the screen. No one is likely to come up with an alternative simply because replicating the functions already baked in to the OS is a little too redundant for anyone to put a lot of time into pursuing.
As far as I can tell there is no "lock screen" optionAs far as I can tell there is no "lock screen" option in Keychain in 10.8 anyway, so your preferred solution is not going to exist if you ever upgrade. Edit: Didn't notice this in Keychain preferences; yes, it is still there in 10.8 anyway, so your preferred solution is not going to exist if you ever upgrade.
If you are using Automator for this can't you just package the script as an app and assign a hotkey combo or leave it in the Dock for quick access?