Revenge plays were familiar to Renaissance audiences. Andrew Gurr, author of Playgoing in Shakespeare's London, points out that the same actor, probably John Hemmings, played both Polonious and Caesar. Gurr refers to the duplicate casting as a theatrical "in-joke." Given this information, how might you reinterpret Polonious' melodramatic line, "O I am slain!" Or would you?
I'm not sure that I would reinterpret that particular line ("O I am slain!"), as it is a pretty serious moment in a serious scene. But I'm sure that Polonius' lines in 3.2, when he says, "I did enact Julius Caesar. I was killed i'th' Capitol; Brutus killed me," would have had the audiences snickering, if not outright guffawing! It would be interesting to know who played Brutus - possibly the same actor that played Hamlet???
Polonius is the proverbial wind bag of the play so I really think this changes things a little bit. After all Polonius admits to Hamlet that he once played Julius Caesar at the university. Perhaps when he is actually slain by Hamlet for being the rat behind the arras eavesdropping on Hamlet's conversation with his mother it is his one final "act". I would compare this theatrical "in joke" to a cross over show of modern day sitcoms where the audience only gets the inside joke if they watch both shows.