A SHVDN script and a RPH plugin have to communicate with each other in a way other than adding a reference to a RPH plugin. Since SHVDN and RPH have their ownAppDomains (RPH creates one for each plugin), SHVDN scripts in SHVDN'sAppDomain can't directly access to data of RPH plugins in RPH'sAppDomains, and vice versa.
I think you could achieve the job with some files or some static memory regions with a mutex or a lock flag (SHVDN and RPH create threads for each script/plugin). Maybe some named pipes can do the job?