OK, not sure there is anyone interested in, as I had so few viewers in more than two weeks, but I figured out a solution, and maybe it could be usefull for someone else, so I'm posting here.
The point is that I cannot access plugin R class, so I have to find a workaround into the method getInflatedBox:
Resources pgResources = null;
Context pgCtx = actContext.createPackageContext(packName, Context.CONTEXT_IGNORE_SECURITY);
pgResources = pgCtx.getResources();
int lLayoutID = pgResources.getIdentifier(layoutName, "layout", pgPackName);
View.inflate(pgCtx , lLayoutID, root);
where actContext is the context of the calling main activity.
Practically I obtain the right ID for the layout I want to inflate obtaining a pointer to the actual resources of the plugin throught the local context pgCtx, built from the plugin package pgPackName.
Hope this helps.