There is no good fix for this. I don't think Windows Update uses a static set of IP addresses, as Microsoft tend to use additional CDNs at times of high load (aka when a new big patch is being released).
I'm not a fan of WSUS. What you could do (if you like a lot of pain) is put a WSUS server somewhere that it can have a public IP address (could be in your colo, in Amazon AWS, etc), and then use the flow preferences to route that public IP address out the DSL interface.
If you enabled AutoVPN failover via the DSL you could also use flow preferences to route the WSUS private IP address over the AutoVPN over DSL.
Another thought (only slightly better than using WSUS) would be to configure a proxy server (such as squid) that is only accessible via the DSL circuit. Then create a WPAD script that sends all requests directly out to the Internet except Windows Update URLs, which you send to your proxy server. WSUS is (IMHO) a pig to administer and keep running, while a proxy server is pretty much automatic.