It's not really considered "etiquette" but rules don't have to followed word for word all the time. However, if there is no reception and they are not attending then yes it would be in very poor taste to send out a registry. If there was a wedding reception and the people you send it to are ones who are invited to it then you MIGHT be able to get away with putting the registry cards inside since there was no bridal shower but be careful with that idea even because some people take it the wrong way as if all you want is gifts. But other than that, it's not a good idea.
You might want to send a separate thing with the registry but I'd say do it by word of mouth, or have a wedding website that has the registry info on it (theknot.com is a great site)