In the end I got back all the
work that had cost so much time and I can play only my incomplete minigames!
Given that I had kept everything from the old console, which I no longer own (NAND, SD backup, movable.sed, otp.bin, boot9.bin*, …), the file that was missing when I restored the MiiPlaza was precisely the "meet.dat".
* this is universal; if you have everything else you should have this too, otherwise you can extract it from the new console.
So, in the right order (not the one I followed, which I put the meet.dat last; but everything works anyway as long as don't launch the Mii Plaza minigames without replacing it, otherwise the saves of the launched minigames are reset and have to be put back), I'll make a quick list of the steps as answer, not a guide, also because the filenames are from the European console:
- I mounted the old sysnand backup with Ninfs, extracted the meet.dat file and restored it to System save with JKSM
- I copied f000000b (still extracted with Ninfs from NAND backup) to "1:/data/<console ID>/extdata/00048000" using GodMode9 (somebody says it might be unsafe, up in this thread, for friendcode or something, but those are details I wasn't interested in, I haven't noticed any problems)
- I restored the Extra data, again with JKSM, which I had actually already inserted from an old dump of mine, but I still followed the steps in the guide you linked to me before realizing that the problem was the meet.dat file
Don't miss any of the three steps: the meet.dat file allows you to recognize saves and contains calendar, world, missions, statistics, etc.; the files in f000000b contain other settings, including your meets, chosen Mii (even if it's not in your current creation center), …; the Extra data files (meet_ext0.dat, meet_ext0.dat, meet_u000.dat, meet_u001.dat, mgBt10.dat, …) contain individual minigame saves (and the related backup).
For those who want to try their hand at it, the references are all on first page of this discussion, in case there are any unclear details on my list.
Obviously for use of the tools, further study will have to be done, if you are not used to using them, but I think I have mentioned necessary tools.
And remember
to do all backups of the new console, before you start,
especially f000000b dir, in case.
Finally, either you work on the old console physically, or you've extracted all the essentials (NAND, SD,
movable.sed, ...) before losing it due to failure/sale/whatever:
no, if you only have the old SD, without all the backups, you'll never get your old plaza back!
streetpass mii plaza certainly fits that description as that is kind of the whole point of having a plaza and not just game data
