I am still having issues with the CustomUUID feature and I believe there are bugs in the RFduino BT stack. If you specify the default RFduino UUID in an Arduino program using:
RFduinoBLE.customUUID = "00002220-0000-1000-8000-00805F9B34FB";
and you use the RFduino test app, it will not see an RFduino when scanning on an Android 4.x phone. It will, however, see it on an Android 5.x phone. If you remove the above line, it works on both.
This is somewhat related to my earlier post about not being able to get custom UUIDs in both the Android app and on the RFduino working. I believe there are hard-coded UUIDs in the bluetooth stack that we cannot see.