If you are allowed to be humorous, you can make an underground newsletter as there should be a receptive audience since the entire court of Denmark is corrupt.  This newsletter would, of course, have to be distributed outside the castle walls.  This would be a satirtic piece, like those of SNL and Mad Magazine.

For example, you could have a headline such as Old Windbag Finally Deflated

Then, this headline is followed by a lead line and a back-up quote in the manner of jounalistic pieces.  Something like--

Polonius, personal advisor to Queen Gertrude, who seems a bit daft herself, hid in the queen's closet while she spoke with her mad son, Hamlet. 

When questioned about the circumstances, the court jester explained, 'It is all very logical.  If you were foolsih and given to much talk, would you not want to hide and listen to a madman, yourself?  Polonius was given to loquacity and probably wanted for new thoughts.'

[Return to the passage in Shakespeare's Hamlet and find incongruent occurrences that you can use to parody, if you may use this genre of satire and parody]

Based on all of the above, concerning Shakespeare's Hamlet, I suggest the following ideas for your newsletter:

• Write it as if you're writing it after Polonius is killed, but before the body is recovered.  Your letter can inform the castle residents as well as the Danish people that Polonius is missing, and presumed dead, but that the only witness isn't cooperating and no body has been recovered.
• You can use imaginary interviews with or remarks by Gertrude, as well as Ros. and Guil., who are sent by the authorities--King Claudius--to question Hamlet, but to no avail. 
• Write that Hamlet is suspected, but not yet accused.
• You can speculate on motives for the killing.

Anyway, those are just some suggestions for you to consider.

I suppose you'd want to first think about your audience, who are they and what should the format of the newsletter be in order to reach them.  Are you looking to make a newspaper type article with headlines to grab their attention like "Prince kills King's advisor" or "Royal Murder behind the Curtains," or are you trying to make more of an announcement for people in the castle already?  Are you going to create a newsletter warning people to stay out from behind the curtains as it has now become a serious health hazard to be hiding behind them, particularly when a certain prince is around?

I think I might be more help if the question was a tad more specific, but at least from what I see, you could really have a lot of fun crafting this into whatever you'd like it to be.  Again, think carefully about the audience, then get creative with how you want to reach them and how you want to use the incident to create an interesting newsletter.