I installed the current Xposed Framework, installed HushSMS and my Xposed module andf rebooted. Then I sent a class0 to one of my other devices to see if it works. The class0 was sent and received correctly but in the inbox of th sending device (with Android 4.4) there was the "allyoumessagesarebelongtous|0|test"
Is this the same problem as yours? Then it's a bug that only displays the message wrong in the inbox but it gets sent out correctly. I will fix this in the next release. Please note that due to the fact that Google removed HushSMS I first need to implement an updated mechanism prior to fixing bugs so all users of HushSMS get notified on a new release. At the same time I hopefully can resolve this bug.
You also said that the class0 was received as normal sms. This can only be related to AT&T. I assume they transform class0 into a normal sms. The Telekom in Germany does this for Type0 and Replace Messages. Have you tried a ping message (Type0 and the others) or MWI?