Yup its working!
I just downloaded USA save file and i'm using it with my EUR backup (rom).
DS save files are definitely NOT region locked and no reverse engineering is required afaik.
So this is what you got to do:
I searched for save file for some time (hard to find it) and only one that I found is this:
https://www.thetechgame.com/Downloads/id=32663/mario-hoops-3-on-3-ds-game-save.html
You need to make an account to download it though (i made it).
Once you download the file from the website, unZIP it. It will be in .DUC format! Not usable for DStwo!
So once downloaded it, go here to convert it to format that DSWtwo can use it (.sav):
https://www.shunyweb.info/convert.php
Click
"browse" to upload your downloaded
.duc save file,
and set
"Source Format:" field to:
"Action Replay Max DS (.duc / .dss)"
and set
"Target Format:" to:
"Acekard 2 | Cyclone........Supercard SD | Top Toy DS (.sav)"
See the picture below:
Than click
"convert" and after that, download your newly converted file
"mario hoops 3on3.sav" file (or whatever its called).
It's a USA save file with
everything 100% unlocked
(i tested it).
Than connect your SD card from DSTWO, to your PC,
and go to where your game is located, and put
"mario hoops 3on3.sav" file you converted in same folder as
"Mario Slam Basketball.nds" rom is, and just rename it from
"Mario Slam Basketball.nds" to "mario hoops 3on3.nds". (same as that .sav file is)
Put SD to DSTWO, plug DSTWO to DS, and play!