You're the first who's ever asked for this! The server browser is a bit janky, at some point it could use some TLC. This is one of it's limitations, caused by one of the features urgently patched on after it was put live, to prevent abuse.
Was actually a feature put in place on request of some of the mass server hosts, who had the servers on private LANs, game traffic punched through via NAT/DNAT.
At this point, it's a good idea, but it's not gonna happen for now. We'll think about putting it on the list for the next time the browser/MP system is looked at. In the meantime, as a utterly-awful work around if you needed, you could proxy traffic through a tiny dedicated VPS, something like a linode nanode, or 'ramnode' looks pretty cheap as well. As long as you got all traffic routed, using NAT it should work fine, with a static ip on the vps end. You can then use server_browser_announced_ip to announce your proxy. As long as browser can get to your actual server, and not take too long getting there, it should work. Make sure to pick a server location closeish to your ISP hub.
Edit: As a bonus, would provide an easy central point to update your IP when it changed. Downside, would need linux 'admin skills.