Comment by bpev
So I haven't integrated FSRS, and of these two, I've only used Wanikani, and have been playing with a reimplemention for Chinese hanzi: https://hanzi.bpev.me
But seeing how it's implemented, I think they could totally integrate something like FSRS to at least just replace their scheduling (how long until an item is next shown). The unlocking system can be implemented as a separate gatekeeping mechanism, and the buckets can be coded for certain step thresholds (instead of wanikani's "stage").
Basically, this is their entire srs system: https://docs.api.wanikani.com/20170710/#spaced-repetition-sy...
I dream of building a WaniKani competitor that uses FSRS. Unfortunately it's one of those projects where 10% of the work is building, and 90% of the work is marketing/community building/evangelism. (And the 10% is not trivial work either!)