Ok folks, I've dabbled a couple times with quart batches under a carbonator cap. Found that this recipe scaled up really well to 2.5 gallons.
Natural Orange Cream Soda
3 12oz Orange Juice Concentrate (No Pulp preferred)
2 Cups White Sugar
1/2 tsp Vanilla Extract
1 Quart Warm Water
2 Gal Carbonated Water
In a suitable container (min half gallon) - mix sugar into 1 qt warm water until dissolved. Add vanilla and all 3 cans of Orange Juice Concentrate (undiluted, straight from the can). Mix well.
Pour slowly into 2 gal cold carbonated water, as it will foam. Close keg, pressurize and agitate keg to mix.
This is inspired by Orangina, but not close to it. I've considered adding a can of LimeAde concentrate or something, but everyone so far has been happy with the results.
I never tried this with yeast, as I was never happy with the off-tastes of yeast-carb'd soda.