All network settings could be managed from the new device's BIOS menu. Profiles could be saved as "Home", "Work", etc.
Legacy DS games could see a virtual, open access point called "default". Alternatively legacy games could see several logical AP connections; named for each named connection that was configured in the BIOS menu ("Home", "Work", etc). These logical connections all bridge to the connection profiles that were configured in the new device's BIOS menu. These could be 802.11a/b/g/n, WEP/WPA/WPA2, and even GSM/CDMA!
Another method would be to suspend the legacy DS title and hook into the new firmware network chooser whenever a single logical AP is chosen.
It should even be possible for the new device to hook into the legacy DS game's code and hijack execution every time the legacy game calls to Nintendo WFC code libraries. This would be the most complicated to implement, but it's VERY possible and would be the most elegant solution.
"Little prigs and three-quarter madmen may have the conceit that the laws of nature are constantly broken for their sakes." -- Friedrich Nietzsche