Subtle changes, bugs and feedback

Looking into it properly, it looks like Facebook do exactly this to protect users from spam/bad links. Link

They describe simply doing this (if you're using jQuery):

function linkShim($link) {

	var dest = $link.data('href'),
		via = $link.attr('href');

	$link.on('mouseover', function() {

		this.href = dest;

	}).on('mousedown', function() {

		this.href = via;

	});
}

$('a.external').each(function() {
	linkShim($(this));
});

Although, what you say about Google's penalisation for it sounds most likely correct — I can't find any decent/official word on it anywhere?

Hmmm...