I think that you can only used this licence for AppMon. You just need to sign up for the free trial license. After 30 days the same key that you have installed will just continue to work. The only limitation is that from now on appmon will only allow "localhost" agent connections.
For Dynatrace SaaS you have a 15 day trial license from the moment you register.
You are right Sebastian...how about the managed license then? Having a lifetime license just to be able to instrument EasyTravel locally would be what I'm after. I love plugin development kind of work and would like to try out new features as they come out. All this work is iterative and an environment that just vanishes every 2 weeks is not conducive to deep problem solving and understanding of the product.