Using AppleScript to open an application without the app stealing focus
(I removed the original script, but I couldn't come up with anything better than this either.)

set fw to front window
do shell script "open ~/Notes/todo.txt -Fa iA\\ Writer"
delay 0.1
tell application "System Events" to tell process "iA Writer"
    perform action "AXPress" of (button 1 where subrole is "AXFullScreenButton") of window 1
end tell
delay 1.5
activate fw


WriteRoom has an option to open documents in custom full screen windows. If it's selected, you could use something like open file.txt -Fjga WriteRoom.

If someone else just needs to open an application on the background, you can use open -jg or launch. launch sometimes opens a visible window if an application isn't open and open -jg if an application is open but has no windows.

set b to "com.apple.TextEdit"
tell application "System Events"
    if bundle identifier of processes contains b then
        launch application id b
    else
        do shell script "open -jgb " &amp; b
    end if
end tell