Well, keep in mind that anything can be argued before a judge. I'm not a lawyer, and there are no guarantees about what someone could try to charge you with. But, considering that disclaimer, I think you were just fine.

Common carriage is a term used to describe a process whereby you transport people or property for hire AND you express a willingness to do it for more than a very small group (one company for example). The grayest part of this rule is the "willingness" part, but I don't think that even applies in your case. As you know, common carriage is not permitted under part 91, and I guess your concern is whether or not what you were doing was common carriage.

But think about it; common carriage requires carriage. You weren't carrying people, you weren't carrying property, and it sounds like you weren't even landing at a different airport than your point of departure.

If you were running back and forth between different airports carrying engineers from more than one company on demand, or if you were hauling broken parts for several different companies, that could be common carriage.

You could have even gone so far as to have carried people or parts etc under part 91 as long as it was for just one company and still been perfectly legal- though it is not too common to see a cessna as the biggest part of a corporate flight department, that is exactly what you could have called it. But, it doesn't sound like you were even doing that.

If all you were doing was taking off from one airport, flying circles to relay, then landing at the same airport, I can't see that anyone could possibly have a case against you. I consider myself pretty cautious when it comes to interpretation of common carriage rules, and I would not hesitate at all to do what you are describing, at least from a CoMyA standpoint.