The problem is that STL strings were not meant to be interoperable with other languages, while SafeArrays are. You could overcome the problem by writing a typedef/struct/class in C++, containing the stl string, and exposing its interfaces to the Fortran program, so that its contents are in effect opaque (but you still have to use those routines to manipulate the string in Fortran).

A rough sketch:

//C++=========
typedef std::array CStrArray;

extern "C" void StrAdd(CStrArray& ar, char* str)
{ ar.push_back(string(str));}

extern "C" void StrSet(CStrArray& ar, char* str, int index)
{ ar[index] = string(str);}

extern "C" void StrGet(CStrArray& ar, int index, char* str,
int strlen)
{ strncpy(ar[index].c_str(), str, strlen);}
...
CStrArray ar;
F90_Populate(&ar);

//Fortran====
subroutine F90_Populate(ar)

integer, parameter:: PSTRARR = INT_PTR_KIND()

integer(PSTRARR):: ar

interface
subroutine StrAdd(ar, str)
!DEC$ATTRIBUTES C, REFERENCE, ...
integer(PSTRARR):: ar
!DEC$ATTRIBUTES REFERENCE:: str
character(*):: str
end subroutine
end interface

call StrAdd(ar, "Foo"C)
call StrAdd(ar, "Bar"C)