I could be entirely wrong but, I do not believe inactive parties can post in bills or organizations they can however, post on the nations page. Now I subject myself to fact-checking on this item but, I know of one example where an inactive party was posting in their host nations page and the moderators intervened stating this was against the rules. I repeat, I subject myself to fact-checking on the last statement. Though in respect to inactive parties being used solely for purposes of spying, which is the more pertinent issue at hand given the context of the thread I second or third George's comments on mine. I find the use of inactive parties being used solely as spies or being used as a means to have a placeholder on an organization to be rather well, I wouldn't do it and while hard to enforce I think it ought be reported if suspected for the moderation team to investigate.
I don't think we need "rules" set down as such on spying. It's a part of this game after all. Each organization and nation has the right to engage in the cloak and dagger intrigues of spying or gaining information if they want. The logic that because one party, organization or nation is doing it and another isn't is hardly grounds to call for a ruling on this. There is a clear consensus from what I've read in this thread on the specific issue of active-parties spying to whatever capacity. The PM issue seems resolved as well. Anything after that is really splitting hairs.