I thought it was a very elaborate fuck you.
I invite you to participate in something with me that I am unable to participate in. That is how much I actually care.
I can't fathom this interpretation? Odo values his time and is a very methodical thinker. How would he get any kind of spiteful satisfaction or feeling of oneupmanship from wasting his time listening to Garak prattle on with pretty lies just so he sit and there and not eat in front of him?
No, the idea is not that Odo would actually eat breakfast with Garag.
The idea is more like saying "You and I will be friends when we eat breakfast together", which will never happen because Odo doesn't eat, therefore implying they will never be friends. This is similar to saying "We will be friends when hell freezes over".
That was my first interpretation of the scene, and I can see that I am not alone. I am still confused though, as I do not remember if the suggestion is being followed up on as is stated elsewhere in this thread. Good thing I am rewatching the show right now and will find out sooner or later.