I've used JKSM (save manager) to reuse old miis while keeping my game progress.
This thread:
https://gbatemp.net/threads/streetpass-2-rise-from-the-ashes.526749/
Describes a different way, where you can backup and share all your unused tags. Somebody is working on an app to help facilitate that and hopefully mix-and-match different games' tags from various backups.
I haven't checked out much of the Homepass/Spillpass stuff, but the one that basically spoofs one of those StreetPass relay points on your own wireless network, still works as far as I know.