For ascending solution taking a lazy approach, it appears to be a trivial inclusion, considering for any n with a solution using 1-9, a solution which includes 0 will simply take the form:

  n = 0 + <ascending_solution_n>

without any additional consideration. Similarly for descending solution:

  n = <descending_solution_n> + 0

The descending case also looks like it'll produce a crap ton of solutions for n = 1 of the form:

  1 = <arbitrary_descending_permutation> ^ 0