I'm using JUCE 3.2.0, and I _think_ the Audio-plugin post-build script for OS X needs to be updated to include use of Plugin AU SDK path, since the tested path won't exist when the SDK-path option is used, and then AudioComponents will be removed from the Info.plist (and that section is where sandbox declarations need to be included).
Hopefully, I'm correct about this .. apologies if I'm not.
# Fix info.plist for AUs built with Xcode 3
if [ -f "$DEVELOPER_DIR/Library/Developer/CoreAudio/AudioUnits/AUPublic/AUBase/AUPlugInDispatch.cpp" ]; then
echo "Removing AudioComponents entry from Info.plist because this is not a new-format AU"
/usr/libexec/PlistBuddy -c "Delete AudioComponents" "$AU/Contents/Info.plist"