United States CIA/NSA, British MI-6, and Israeli Mossad/Shin-Bet can do the honors of assembling the list. Final List be reviewed and approved by, I would suppose, SecDef (or his appropriate lieutenants).
and then a means to prove that the kill has occurred,
Required Terms of Proof (perhaps photographic, perhaps physical evidence -- this is war, there's no excuse to be squeamish) may be specified by SecDef as condition of Bounty.
and that may be difficult if in a hostile zone.
Indeed. And that is a strong argument for permitting acceptance of these letters of Marque and Reprisal by ex-military, ex-intel, or ex-security mercenaries (as I suggest) and refusing their acceptance to, say, dogcatchers (also as I suggest -- clause, #1: "excepting those contractors whose services the Department of Defense shall consider prudent to refuse")
Assuming the US is willing to get its agents to go down and dirty, this free lance scheme seems to involve risks that exceed the benefits.
The private mercenaries going after righteous Vengeance and a half-million bucks per terrorist head (rough average) are entitled to be the judge of that, don't you think?? Nobody's drafting them, this is (by definition) a volunteer-only operation. Ross Perot 1979, albeit search-and-destroy, not seek-and-rescue (Yes, Perot's op was a little messier in execution than most people realize. But that is to be expected. The bottom line is, they went in, fully accomplished the mission objectives, and got out. Fact.).
I doubt Islamic nations will tolerate free lancers wandering around in any event,
Who said the free lancers are going to ask anybody's permission? Clause #5: "in the case of capture by a hostile agency, the Department of Defense shall have the right to disavow any knowledge of, or responsibility for, the contractors in question".
Registered Contractors go in with no implied safety net. They know this. The opportunity for vengeance, and the money, are freely offered; so are the risks.
while they would have to accommodate US agents.
Sure they'd "accomodate" official US government agents (somewhat) -- Although if Al-Qaeda really does have 20,000 followers world-wide, "official" US Counter-Terror just got stretched real Freepin' thin. Whaddya suppose message we're gonna send to over 1,000-2,000+ individual terrorist cells worldwide? "Please take a number, US Government Counter-Terror Ops will get to your case eventually??"
Meanwhile, the Islamic "host country" security service guy assigned to act as liaison to US Dep't. of State makes a phone call tipping off his cousin's brother-in-law (who is the local Bin Laden cell leader) that US Counter-Terror is going to swing by his safe house in 30 minutes... and he's gone like the wind.
Free-lancers, on the other hand, are not going to be constrained by the usual red tape... and besides, free-lancers aren't Mutually Exclusive to "official" US Counter-Terror ops anyway, they're just coming from a different level, and from a different direction.
And it seems to me that's a good idea, as the Battlefield doesn't seem particularly "two-dimensional" anymore after Sept. 11, 2001. We're not going to win a 3-d chess game if we only play a single-level game.
I've gotta prayer meeting before Sunday Evening Church tonight. See you after the service.
I'm not sure that you understand how deep cover government agents operate. They don't have a host government nanny. But if they get in trouble with the host government, the host government would be told to lay off.