If you use this:

 System.Reflection.Assembly.Load(System.IO.File.ReadAllBytes(path))

It will not lock the file.

Edit

While this works, it is not the best solution, but the better way of doing it is a lot more involved (too much for just pasting all the code here for it).

I have created a public repository on Github here with all the code for doing this properly:

Loading Assemblies without Locking by using Shadow Copying.