So how do you make an acceptable representation? Since the look of baby's breath is lacy and relies a lot on the space between the flowers, just do tiny flowers that are on the ends of wires, then attach them together.
I'd done a cake once that had a bunch of baby's breath type filler, and I did those by just piping royal icing onto the end of wires. For this cake I wanted to recreate the rounded shape of the entire flower, though.
I took some floral wire and twisted it so that it was in an umbrella shape, with the ends of the wires forming the spokes. I made a bunch of teeny flowers using fondant and a mold, and let them dry. Gumpaste probably would have been better, because the next step involves hot sugar.
I took the tiny flowers and put them all upside-down on a foil-lined tray. Then I cooked sugar to 290 degrees and dipped the little wire umbrellas into the sugar. When they were coated I pressed the wire onto the backs of the flowers so that the entire umbrella was covered with flowers. The spaces that showed would be visible since the wire showed through the cooled sugar, but they're good and stuck to the wire. The open areas give you the look of the baby's breath, with the little flowers and green space between them.
If you don't cook the sugar hot enough it can soften up, so be careful. If it's humid it can also soften up, so this probably wouldn't be the best way to do it if you're dealing with a lot of rain.
Another way to do this would be to use royal icing to attach the flowers, or just to pipe the royal onto the wire. I personally can feel the hand cramp just thinking about doing that. If you have strong hands that don't cramp up, royal might be a good option.
Kara Buntin owns A Cake To Remember LLC, custom wedding cakes in Richmond, VA