Mark Kleiman says let 'em fly:
So call him in, giving him immunity from proseuction for anything he might say (thus barring any Fifth Amendment claim), to ask him whether he was covering up for Cheney and/or Bush when he perjured himself. Let Bush assert Executive Privilege and "direct" him not to appear, and then go argue in court about that. If Fielding tries to raise the question of what legislative purpose is served by the inquiry, the answer is simple: we're trying to figure out whether to impeach Cheney.
Meanwhile, I'm also inclined to agree that "haul Sara Taylor off to the slammer" is premature since it seems that there's going to need to be some litigating one way or the other, and having her wait that out in prison only makes her look more sympathetic.