Wait a minute, but I take it the DS's local wireless comm still hasn't fully been cracked ?*
*As in, still requiring /two/ Flashme'd devices ?
You might be mixing two things up.
Download play is the stuff that flashme (or equivalent) tends to be looked for to handle. This is what allows people to download RSA signed things from the download stations and multiplayer games that send a client out for others* to play (usually cut down but sometimes interesting and unique) versions for. There were some attacks on this for the download station at least (
https://gbatemp.net/threads/haxxstation-ds-download-station-exploit.473648/ ), and technically wifime was a bypass for earlier hardware revisions.
*If you are poking around inside a DS game this part of it is usually another DS file (as in open up in whatever you used to poke around the base game). Typically a file called utility.bin and located in a folder called dwc, or a file with the extension .srl (srl being the internal compiler extension of choice --
http://problemkaputt.de/gbatek-symbolic-debug-info.htm ). Sometimes quite nice to look at, especially if it is a cut down version of the main game and you only want a few art assets, font or an idea of how the main game works. Naturally anything that can be compressed will then be compressed, but such things are known and easily dealt with by the many tools to handle DS compression formats.
nifi is the term most use to describe the custom (in reality a headerless standard 802.11b packet/comms) local communications which things like commercial games, pictochat and whatnot use to allow people to play multiplayer without having to join wifi networks to have them host. Have been able to run hacked ROM, cheats and have custom packets if you could convince it to send one from the point where custom ROMs could be run without restriction beyond whatever the games themselves had for sanity check/anti cheat (and I am not sure I ever saw any for local multiplayer, mostly just pokemon doing wifi and whatever games had for anti cheat/cheat frustration in single player that carried over there).
I am not aware of any nice libraries that people can use for homebrew to do much the same, hence people mostly going for local LAN options where the would be players connect to a router. I have not been following things for a while so someone might have made something in the time since, though I do follow homebrew in general so would like to believe I would have seen more people use it if it was a thing.