We followed up on this one in e-mail, for others here are the results.

For now if you need to convert a non string input to a string output, you'll have to implement the code in both C# and Arduino. To do this, on your node click on the double red arrow icon, which will show a tab for C# and Arduino code.

In the c# code you can use the ToString method like:

Output_String = Input_Integer.ToString();

For Arduino you have to cast to a string like this:

Output_String = (String)Input_Integer;

I've added doing this automatically to my todo list so you can use .ToString and it'll change it for the Arduino code.

If you need to convert a number to a string I would make a node that does just that, and feed the converted string value into your custom node so you don't have to implement the whole custom node in both languages.