Not checked on 1.6. However, as mentioned earlier, it appears that it is a latent defect(if this is accepted as defect by R&D), since it is not working SoapUI OS 4.x and Ready API 1.5, so I would expect that very few chances that it could work in 1.6 too.
AfterLoadScript is being fired when loading a project with built-in project settings. The -P argument changes a property only after a project is loaded. I’ve consulted with our Dev Team and they suggest that you add your assertion to project’s Setup Script. The code located there will be executed before loading a project. Please let me know if this solution works for you.