Have you experienced the %variant mode in Bison development version?  It works well, and avoids this nasty POD issue.  Have a look at http://www.lrde.epita.fr/~akim/download/bison-2.4.570-7a582.tar.bz2.  Look for variant in the documentation.  The following example shows what you can do.  I need to polish and publish this :(  Help would be most welcome.

%type <::std::string> item;
%type <::std::list<std::string>> list;

%%

result:
 list  { std::cout << $1 << std::endl; }
;

list:
 /* nothing */ { /* Generates an empty string list */ }
| list item     { std::swap ($$, $1); $$.push_back ($2); }
;

item:
 TEXT          { std::swap ($$, $1); }
| NUMBER        { $$ = string_cast ($1); }
;
%%