[rolian]

Given that you have the OF entry in Services, then you can highlight in Finder the file you wish to send to OF, hit your LaunchBar InstantSend keystroke, and when the file shows up in Launchbar, hit tab and then Omnfsinb or whatever letters you need to call the Omnifocus-Send to Inbox service, and boom, you will have the quick entry panel open with your desired file in the notes section of the task. You can assign your own abbreviation to the Omnifocus-Send to Inbox service. If you make it short, once you have your file in Launchbar (either directly or by using InstantSend), it is a tab and a couple of keystrokes if you use a 2-letter abbreviation for the service. Don't think you can get much shorter than 3 keystrokes, if I understand what you are trying to do.

Note that you may need to be sure LB is indexing the Services folder to make this work. I don't recall if I had to force that, or if LB does it automatically.