can't make a string literal type

Putting aside the looping instantiation, you cannot achieve what you want as a function parameter cannot be used as a constant expression, which is required of template arguments. Meaning the following is not allowed either:

template <typename... A>
constexpr auto make_slit(A const... args)
{
  return slit<args...>();
}

// error
make_slit('a');

If you find this surprising, keep in mind that constexpr functions are a feature to allow some functions to also be usable in a constant expression. Yours isn’t in general however:

char c;
std::cin >> c;
// what is the result type?
make_slit(c);

I should note however that during the design of literal string operators it was suggested that a function template form be allowed (much like they are for integer and floating-point literals), which would achieve exactly what you need:

// allowed...
template<char... Cs>
constexpr slit<Cs...> operator"" _slit()
{ return {}; }

// ...but only for
auto constexpr s = 123_slit;
auto constexpr t = 12.3_slit;

// ... and not
auto constexpr u = "abc"_slit;

This missing functionality was brought up most recently in 2013 with Richard Smith’s n3599: Literal operator templates for strings. Unfortunately I don’t know what the current status of the feature is.

You can find a solution to expand a string literal to a parameter pack here

#include <iostream>

// c++14 has it http://en.cppreference.com/w/cpp/utility/integer_sequence
namespace detail {
    template <int... Is> struct seq { };
    template <int N, int... Is> struct gen_seq : gen_seq<N - 1, N - 1, Is...> {  };
    template <int... Is> struct gen_seq<0, Is...> : seq<Is...> { };
}

constexpr size_t operator"" _len ( const char*, size_t len ){ return len; }

template < char... val > struct slit {};

#define EXPAND_STRING( type_name, str ) \
template <int... Is> slit< str[Is]...> make_##type_name( detail::seq<Is...> ); \
using type_name = decltype( make_##type_name( detail::gen_seq<str##_len>{} ) );

using Manual = slit< 'b', 'a', 'z'>; 
EXPAND_STRING( MyFoo, "foo bar baz");
EXPAND_STRING( MyBar, "bla bli blo blu");

inline std::ostream& operator<<( std::ostream& os, slit<> ) { return os; }
template < char first, char... chars >
std::ostream& operator<<( std::ostream& os, slit<first,chars...> ) {
    return os << first << " " << slit<chars...>{};
}

int main() {
    std::cout << Manual{} << "\n";
    std::cout << MyFoo{} << "\n";
    std::cout << MyBar{} << "\n";
}

EDIT : replaced the constexpr strlen with a custom literal, it returns the length directly and remove the dependency with relaxed constexpr function of C++1y.