Thanks. Not the answer I wanted, but at least an answer. Here is a simple AppleScript that handles this case pretty smoothly in case that's useful for people using the AS API instead of JS.
- tell application "LaunchBar"
set _selection to selection as text
-- In case things are slow, hold until LB has focus
repeat 10 times
if has keyboard focus is true then
exit repeat
else
delay 0.1
end if
end repeat
end tell
tell application "System Events" to tell application process "LaunchBar" to tell window 1
keystroke space
delay 0.1
keystroke _selection
end tell