There is existing functionality to perform this navigation. It is one more click if you're navigating from the questions list, but I think it's a lot more functional to rely on that than to introduce a break in consistency for the username links on the site. I actually find utility in navigating to the profile from the front page and the questions list, but that may just be me.
When looking at a question that has had activity, the right sidebar includes a field called "last activity". Its value, which for new activity will generally be "today", is actually a link. Clicking on it will skip through the question and direct you straight to the most recent activity that you are seeking.
If you're on the front page, this is even faster. Clicking on the timestamp next to the username will navigate to the same URL as the last activity, so you'll basically be brought to the post from that user. This functionality is not present on the questions list, but that sounds like a proper feature request on its own.