if you go to versethis.com they have some good ideas.

No one will be offended, I promise. I've been to plenty of weddings like that and people generally understand the constraints of space.

I wouldn't go to great lengths to apologise or explain to guests why they're only invited to the reception and not the ceremony. Just something short, sweet, and cute. To those only getting an invitation to the reception, I would suggest using wording that would make them feel like it is the most special reception and they would be the most cherished and special guests to you for coming. Flattery always works best if you ask me

We did the same thing - here's what we did (of course you can change the words around to add your personality, but you get the idea)...

Mr. and Mrs. PARENTS OF THE BRIDE
are pleased to announce the marriage of their daughter
BRIDE
to
GROOM
son of PARENTS OF THE GROOM
on DATE
The pleasure of your company is requested
at a reception held in their honour
at TIME
LOCATION
ADDRESS
CITY, STATE

Hope this helps!

The wedding was 3 months ago so we're good to go...but thanks!