Simple C# app which runs whatever the user specifies. Place in System32, then in WSL do `alias open='open.exe'`

Program.cs

using System;
using System.ComponentModel;
using System.Diagnostics;
using System.Linq;

namespace Run
{
    class Program
    {
        static void Main(string[] args)
        {
            string exeName = Process.GetCurrentProcess().MainModule.Mo‌​duleName;

            if (args.Length < 1)
            {
                Console.WriteLine($"Usage: {exeName} <command> [parameters ...]");
                return;
            }

            var ps = new ProcessStartInfo(args[0]);
            if (args.Length > 1)
            {
                string[] newArgs = args.Skip(1).ToArray();
                ps.Arguments = String.Join(" ", newArgs);
            }

            try
            {
                Process.Start(ps);
            }
            catch (Win32Exception ex) when (ex.Message == "Element not found")
            {
                Console.WriteLine($"Error opening file {args[0]}:");
                Console.WriteLine("No default application has been registered for that type.");
                Console.WriteLine();
            }
            catch (Exception ex)
            {
                Console.WriteLine($"Error starting process {args[0]}:");
                Console.WriteLine($"{ex.Message} ({ex.GetType()})");
                Console.WriteLine();
            }
        }
    }
}