How can I create a custom Property for my assembly?

Your compiler does not support it. AssemblyFileVersionAttribute and all work because the compiler generates a win32 resource in your assembly called VERSIONINFO with that information in it. That is what windows is reading. Other attributes are just saved as metadata along with your code.

You could make a tool to change the VERSIONINFO resource after you compile.

Its not the answer yet, but one 1/2 way hack is to use:

[assembly: AssemblyDescription("One Line of Content Here")]

And that shows up under "Comments".

(still researching, this is something I've wanted for a while.)