or you can make the folder a popup tab. folder actions will work on tabbed folders because they’re considered “open” by the finder. here’s a script i used to attach to a folder awhile ago:
on opening folder theFolder
tell application "Finder"
activate
open selection
set size of container window of theFolder to {240, 600}
set position of container window of theFolder to {800, 870}
set popup of container window of theFolder to true
set pulled open of container window of theFolder to false
end tell
end opening folder
on closing folder window for theFolder
beep
tell application "Finder"
activate
open selection
set size of container window of theFolder to {240, 600}
set position of container window of theFolder to {800, 870}
set popup of container window of theFolder to true
set pulled open of container window of theFolder to false
end tell
end closing folder window for