We are doing the same thing and wording the seperate invite to the reception only:
that we are going to be married in a private ceremony and that the people are invited to celebrate with family and friends at the reception. It is worded more formal than what i just wrote but no one seems to be offended by it. We did a totally different invite to the reception only. We got a seal and send from www.now-and-forever.com.

Here are a couple of suggestions:

Lisa and Gregory are getting married June 6th. Please help us continue the celebration by attending a reception in their honor (date, time, etc.)

Or

Please help us in celebrating the marriage of Lisa and Gregory. A reception will be held.....

Hope this helps!