At the risk of angering those who have produced these materials — which are wonderful for the languages they were produced for —

My observation has been that trying to use existing TPRS materials written for other languages (Spanish and French, primarily) to teach Chinese, Japanese, etc. does not work well for languages that are structurally significantly different from the original language, even when they are “adapted”. To truly adapt a curriculum requires re-doing it from scratch to consider what structure you are going to focus on in the new language. That usually doesn’t happen in these situations, because logically the idea of “adapting” something is to use what is already there in large part.
TPRS materials for the Spanish/French/etc. languages have to focus on conjugation. There is not much conjugation difficulty in Japanese. There ARE issues of register (-masu versus plain forms) and of course the semi-opaque writing system. Those are very different challenges than what Spanish acquirers have to deal with. And of course, you have the lack of readings — opaque writing system languages require a lot more reading practice and actual reading instruction, versus phonetic alphabetic languages where if they can see it, they can read it, pretty much. (TPRS levels, assuming no diglossia.) Adapted materials typically take the old view that you can’t learn kanji until you’ve been able to read in kana for some time, which doesn’t seem to be the case based on our work in Chinese with cold character reading. So eventually the backlog of unseen kanji will come home to roost as well.
I would guess that Japanese, like Chinese, can be acquired through TPRS to practical structural mastery in 2-3 years. Spanish takes longer — all the tenses and all the persons, plus the moods. (NB: I know some Japanese but only enough to be irritating and dangerous, not enough to be able to really “get” how to organize a Japanese curriculum.)
My feeling is, if a language “works differently” (not really a technical term there, but people who speak such languages will probably know what I mean) it should have its own curriculum written just for it. The question becomes (in the absence of such a curriculum) does the benefit of starting with something (not having to choose items, make up basic stories) make up for the effort of having to write supplemental readings and deal with the “discomfort” that an adapted text can sometimes bring? Or would it be better to start with one’s own language-specific item list and create stories that way? That decision depends on the individual and his situation. And maybe even funding. It takes FOREVER to write a complete year-long curriculum with readings.
But I would totally take a TPRS Dothraki class.