With Zen Cart you can offer free or discounted pircing based upon the quantity purchased. However, this is for a single purchase - X amount purchased at one time.
~Roland
Are you talking about like the GoRewardPlus program that GoTruckStop.com offers?
You can find it at their front page at http://gotruckstop.com , then selecting "Programs" and the Go Rewards Plus program ... or just click on here ... http://GoRewardPlus.com
They offer just like what you were talking about.
You stated ... This would require a good shopping cart, and I`m wondering how hard it would be to implement?
You mean something like AppleCart, that GoTruckStop.com and BoxerHaus.com both use?
It sounds like GoTruckStop.com and you are on the same page. Imagine that !!
Gee, it looks like you just might be starting to realize why several folks have been recommending AppleCart !! 
The AppleCart Shopping Cart does exactly what you are asking for - keeping a customer profile, all the purchases that a customer makes, and allows you to enter a "Discount Percentage" in the customer profile also.
Why not contact GoTruckStop to find out if it really does work the way I`ve been talking about?
Obviously, GoTruckStop.com was hot on the concept many months ago, as GoRewardPlus.com appears to have been registered last year.
Its amazing what you might discover when you start to look under the covers, eh? 
Gee, your osCommerce & ZenCart folks can`t do what you`ve asked for as both JerkyJanie and Roland have posted.
Maybe its time for folks to taking a long hard look at AppleCart as a viable solution? Unless some have an underlying motive NOT to take a look.
I`m a beleiver that you always do whats in the best interest of the client - not whats in the best interest of you. If you follow that principal - success is bound to come your way.
I don`t understand what the "motive" would be, not to look into another cart if needed. I`m sure it works for some.
The rewards program sounds good, but we handle specials and incentives in a different way that works for us. Everyone should use the programs and carts that work best for them.
Even though we are a B2B company many of the ways to deal with customers are the same. Our company’s product prices tend to be on the low end for our industry and we have a lot of unexpected expenses to deal on a regular basis with so we cannot afford heavy discounts. As inflation continues to take it toll and we are force to increase our prices. We have found that offering our current customers a limited time offer at the old price just before we increase our prices makes them very happy with our company. I would think something like this would work well for retail too.
Just a thought…
Imagine that !! And I thought that ZenCart was the "cat`s pajamas".