A handful of companies have policies like "business class is ok for flights over 5 hours (or 6, or 8, ...)". A different handful use business class for their top executives, who can actually work while on the flight and in the lounge, and who make 10 times your hourly rate, or perhaps 100 times.
Clients can be very strange about hourly rates and travel expenses. They generally expect you to follow the policies their staff follow. You're welcome to take on only clients who will buy you business class tickets, but you may find that's more of a way to turn down travel than anything else.
Also, not everything needs to be pre-negotiated. When they say there will be occasional travel you say something like:
I have no objections to that, assuming it serves the needs of the project and I don't incur the expenses. I don't normally bill for the time I spend traveling and as a result I have fairly strong opinions about things like connections, which airlines I fly and so on. Do you have policies you'd expect me to follow? I'll keep those in mind while we're deciding about any trips that might need to happen.
Months later, when they want you to fly somewhere, that's when you can start with "I would prefer airline X" or "I never change planes in Chicago" or "of course I need to go business class." At that time they are making a concrete decision for a concrete cost and benefit. While they are still a potential client, your "business class only" policy has unlimited upwards cost for unknown benefit. Once the project is underway, your "direct flight on airline X in business" request will cost $1000 and it's what is needed to have their valued consultant in the room. It's far more likely to be acceptable to them. If it's not acceptable to them, you will know how important it is to you, and can make your own business decision accordingly.