Jimmy Carter has always self-identified as an "evangelical protestant" and I would argue he's well to the left of the "marxist" Pope. (I've never seen Jimmy denounce abortion or gay marriage). I'm pretty sure leftists would be delighted if ex-President Jimmy addressed Congress.
Of course, Jimmy's not a minister. But leftists would also be pleased if Jim Wallis, Michael Dowd, or Rob Bell (all of whom identify as "Evangelical protestants") addressed Congress.
On the other hand, if a CONSERVATIVE "evangelical protestant" were to address Congress, I agree that leftists would rant and rave about it, and scream about separation of church.
If you convince your Congressmen to get such a figure to address Congress, more the power to you. As you noted, you can probably find at least 3 or 4 protestant faith leaders with enough prominence to tie up traffic. As a Catholic I'd be fine with it.
Of course, given the historic ratio of dozens of protestant congressional chaplains compared to only 1 or 2 Catholic chaplins, and dozens of protestants presidents but only 1 Catholic president (JFK, who wasn't the heroic conservative figure a bunch of freepers like to pretend he was) I can't really feel bad that there's been more prominent Catholic clergy than protestant clergy addressing a joint session of congress.