In reply to DrSuperEvil:
I have now managed to get the Barrack to host the Rain Actor via the Hosting: Host - Subject field. The Rain appears above the Barrack when construction starts.
But I think I now what the major problem is....
Same problem with the Persistent effect with the Unit Order Queue - the effect starts when you start building but is destroyed when you stop. It is never created again when you resume the construction.
Any idea how to check when a construction resumes?