[Update: So it seems like they made good on their promise, at least in my particular case. After signing up for the newsletter at the beginning of the month I obtained an email this week with the Super Butoden code. I can confirm that the code does work on North American Switch accounts. You can find the reward letter in the gallery below.]
So the day has come: Dragon Ball FighterZ is out on Switch, and the pre-order campaign is over for Super Butoden. Yep pack it in folks, Bandai Namco said that there isn't recourse for getting Butoden unless you pre-ordered FighterZ: not no way, not no how. Wait, there is a way?
Just three days after FighterZ's arrival on Switch, the Bandai Namco Latin American Facebook page announced that if their newsletter reaches 10,000 subscribers, then they'll give away Super Butoden download codes. Note that no specifics were given, so they have to reach the number anyway (or say they reached it after getting a ton of free subscribers), and then they'll give an unspecified amount of codes away. As a reminder Butoden is not translated.
We've reached out to Bandai Namco for clarification on whether or not this promotion works worldwide, and for more details. Either way if it does go through and you do get a code, it's super easy to make a Switch account in any region to redeem it.
Bandai Namco [Facebook]