Hey Phil,
It's been a while since I've tested them (and you may already know this), but for starters, anchor links have not historically worked reliably across all email clients. That's not a reason to avoid them altogether, but you definitely have to "proceed with caution" when using anchor links in email. They aren't a substitute for making your emails shorter and more targeted so that it's easier to prioritize the right content to the right recipient.
That said, I don't think it's possible to track clicks to email anchor links. I haven't tested this, but I would guess that Marketo doesn't even encode these links for tracking, since they aren't referencing an http:// or https:// URL in the href tag. Anchor tags, by design, don't call a server in order to deliver content (like a landing page) - they describe behavior that takes place client-side within the browser or email client.
In a browser environment (e.g. on a landing page), it's possible to "tell" your server that someone clicked on an anchor link by calling a javascript event when the anchor link is clicked. That's not an option in email, however.
Hope this helps!
-patrick