As Prof. Colin Miller TG points out in a new post, Fed. R. Evid. 804(b)(6), applies to both sides. The Mil. R. Evid. contains the same language.
For an example of a case in which the government forfeited its right to object to the defendant’s admission of hearsay from a declarant whom the government rendered unavailable, consider the recent opinion of the Ninth Circuit in United States v. Leal-Del Carmen, 2012 WL 4040253 (9th Cir. 2012).
Prof. Miller concludes:
Because the court thus found that the government caused Garcia-Garcia to be unavailable for trial and had the intent to render her unavailable for trial, it concluded that forfeiture by wrongdoing applied: "Because the government was responsible for rendering Garcia–Garcia unavailable as a witness, admission of the videotape would prevent it from benefiting from its own wrongdoing."