I am not sure whether this belongs here or in jan nasa ... But, in any case, whatever basic POS we put 'taso' into, it also must function as a preposition, meaning "except (for), but". As such, it attaches mainly to NP, especially to 'ala' and 'ali' and to collective uses, including 'kulupu'. If you except this initial position, the remaining issue is how the PP groups into the NP. For "Everybody but Bob came" can it be simply 'jan ali taso jan Papi li kama' or must it be 'jan ali pi taso jan Papi li kama' or even 'jan pi ali pi taso jan Papi li kama'. The 'pi' rules seems to rule out the first, though the case is special enough to allow an exception, perhaps. Various readings of the situation then argue for one or the other of the remaining cases: is "all except Bob" a special quantifier on 'jan' or is 'jan ali' a complete reference that is then modified? Lojban me says the former; practical me says the latter is good enough -- and shorter.
Any tought, including especially cases that shed some light. And also any cases of a 'taso' PP that is not in an NP.
I am unsure about the significance of the fact that you cannot detach 'jan ali li kama' but you can detach 'jan Papi li kama ala' (we don't have a way to say "almost" that I know of, though both 'ken' and 'poka' have been suggested and maybe used).