It is! GBARunner2 is a DS homebrew hypervisor that can run (a lot of) GBA games in DS mode [1]. It can e.g. be used to run GBA games on the Nintendo DSi, which lacked the GBA slot and AGB mode, or run GBA games directly from a DS flashcard.
> and patch the GBA Link Cable serial comms to instead use the Nintendo DS's Wi-Fi connection.
It isn't - practically at least. There have been attempts and it works as a proof of concept with special builds of GBARunner2 [2]. Sadly the game logic in almost all GBA games does not appear to accept the huge added latency for wireless connections, compared to the almost nonexisting latency for a serial cable. This is also a problem for GBA emulators like mGBA, which only support game linking on the same computer and not e.g. on the network [3]. Games made only for the Link Cable (and not the niche Wireless Adapter) just can't deal with network latency.
[1]: https://wiki.gbatemp.net/wiki/GBARunner2 and https://github.com/Gericom/GBARunner2