So far this is what I found.
If you decide to buy the bike that you paid for the domo/rent of the bike will be reduced in the price when you buy it.
Santa Cruz (2017 demo bikes just in being built)
http://www.jbssport.no/ Yt industries which are sold for 20% off later.
On utflukt you have to buy a festival pass. the brands vary from year to year,but canyon has been demoing there for few years.
Demo days In Trysil in June 2017
Trysil 22-25. Juni 2017
Check if a bike park got current models of bikes,but you are lucky if they do,and usually they have the cheapest models, and after few years they replace them with more current bikes.
Some biek parks got demo days:
Kjerringåsen alpin senter had demo days in the past,and will probably have more.