Well, first of all I like your unofficial official policy. Working in the world of software I understand where you're coming from. I've gotten more clients by showing them out of the box solutions (eliminating my or anyone else's maintenance costs).
That being said, I would be willing to pay for major versions. Having a policy stating that you may charge for major version upgrades, would give you an "out" so you wouldn't have to charge, but could -- and not feel guilty about it. Then you could go along your merry way and if you had a major version upgrade to offer you could announce it as free or paid and it's fit into your policy. But I guess that's what you current unofficial policy is doing -- it just doesn't give any sort of expectancy. I'd rather expect to pay and then be pleasantly surprised by a free upgrade, than expecting a free one and be surprised by a required payment.
Just some more food for thought! ;)