Ok, then I think you can either bite the bullet and write the function with the horribly long signature, or you can keep the version with the variadic template signature and leverage jitting via a Define + some action that triggers its evaluation at every entry, e.g. Sum:
gInterpreter->Declare("#include <header_with_variadic_helper_function.hxx>");
df.Define("dummy_col",
"myfunc2(v1, v2, v3, v4, v5, v6, v7, v8, v9, v10)")
.Sum("dummy_col"); // this Sum forces evaluation of dummy_col at every entry