You can see in torch/csrc/jit/api/module.h that torch::jit::Module::forward is implemented like so:
  IValue forward(std::vector<IValue> inputs) {
    return get_method("forward")(std::move(inputs));
  }

You should be able to call exported functions like so:
auto mod = torch::jit::load(...);
auto exported_method = mod.get_method(<exported_method_name>)
auto result = exported_method(...);