the simpler way to do is by means a visit of the tree, 'hardcoded' on the symbols you are interested.
Here is a more generic utility, that uses (=..)/2 to capture a named part of the tree:
part_of(T, S, R) :- T =.. [F|As],
( F = S,
R = T
; member(N, As),
part_of(N, S, R)
).
?- part_of(s(np(det(the), n(man)), vp(v(went), np(n(home)))),np,P).
P = np(det(the), n(man)) ;
P = np(n(home)) ;
false.
It's a kind of member/2, just for trees. BTW I don't understand the first part of your question: why do you want to use phrase/2 on a syntax tree ? Usually a grammar (the first argument to phrase/2) is meant to build a syntax tree from 'raw' characters stream...