I like that idea a lot. I do have a concern however. If it displays a discount code it would most likely be a generic code so it could easily be shared. I suppose I could block the codes from being posted on the site through buddypress forums and what not.
I'm trying to think of the simplest way to do something like this. I don't want to really edit any core files of plugins so I'm thinking about a plugin that would detect the unlocked achievements the user has received that could simply let them use the discount code in wp-ecommerce. Otherwise it blocks them from submitting it to the form or somthing. That way it doesn't really interfere with the wp-ecommerce plugin itself and one code could be used even if users send it to others. It's a bit of a work around, but it seem like a possible simple solution.
That could be a really bad idea, but it's the first thing I thought of just now. ha.