The libraries reward point feature would not really be suitable for the functionality you have outlined as it is intended to be monetary based.
However, there would be nothing stopping you creating a new discount every time the user placed an order that was linked to the users id. The discount could then be defined with a voucher code so that the user can then redeem the voucher when they wish.
Upon creating the discount, you could check whether the user had an existing discount, and then just up the percentage as required.
Whilst this is possible, I will advise you that incrementing the discount as you have described without any other conditions would be a very bad idea. All I would need to do is place 50 separate orders for the cheapest item on the site. This would increment my discount to 100% (50 orders x 2%), I could then place another order for everything on your site with a 100% discount - free!
So just make sure you have some conditions capping the percentage at a maximum limit.
