Is it possible to jury-rig Internet Sharing to share an internet connection wirelessly, when my Mac already connects to the internet wirelessly?
In short, not likely to be possible using a single Airport card.

99% of the time when a workstation's WiFi card connects to a standard wireless access point, it does so in WiFi "client" mode, which has certain physical (i.e. radio-frequency) requirements. 

When operating in "client" mode, your workstation's WiFi card cannot act as a repeater or quasi-router for other clients. ("Ad Hoc" wifi mode can support multiple simultaneous connections to different devices, but Ad Hoc and Client modes are mutually exclusive.) 



What should work fine is for you to add a second, separate WiFi interface to your Mac, for example a USB-to-WiFi adapter. (cheap, $19! And no, I don't work for OWC.) 

That independent WiFi interface can then be separately configured for Internet sharing to give your DS (or other devices) a "temporary hotspot" connection. USB 2.0 has a minimum latency of 0.125ms and an average bandwidth of 25 to 35MB/sec -- probably plenty for your gaming needs, unless you're going to be connecting more than 3 or 4 clients.

Also, this question is closely related to Wi-Fi Connection sharing on Macbook Air via USB dongle , which is still unanswered.