I would do the oil changes every 7k miles. Each time you top off, you are adding fresh additives and oil, so your oil is fresher between the longer oil change intervals.
I'm tempted to see it this way as well; low consumption just gives the impression of a better-made or healthier engine, but I've learned here and elsewhere that's not necessarily the case. For example I had an E30 325i that used about a quart every 2500 from the time I first owned it until I sold it over fifteen years later with 365k on the clock. Compression was always consistent across all six cylinders, the oil burn was also consistent, fuel economy was excellent and performance exactly what it should have been. It was just normal for that car.I tend to lose respect for an engine that consumes up close to a quart per OCI, so it's going to get Supertech FS at about an 8k OCI and maybe even a few ugly looks as I pass by.