Indeed, this is currently possible because all of the graphs (and their instances) share the same JVM. However, there is a high chance that this will change in some future version in order to increase stability of CloverETL software. Moreover, singletons are not officially supported, so in case of any problems, we won't be able to help you. Due to all these facts I must recommend against using such long-living class instances.
If you share what you want to achieve, maybe we will be able to find more suitable solution.