I'm not 100% on the effectiveness of this, but I implemented an extension method to SecureString based on the solution in this article.

$TypeDefinition = @"
using System;
using System.Runtime.InteropServices;
using System.Security;

public static class ExtensionMethods
{
    public static string ConvertToUnsecureString(this SecureString securePassword)
    {
        if (securePassword == null)
            throw new ArgumentNullException("securePassword");

        IntPtr unmanagedString = IntPtr.Zero;

        try
        {
            unmanagedString = Marshal.SecureStringToGlobalAllocUnicode(securePassword);
            return Marshal.PtrToStringUni(unmanagedString);
        }
        finally
        {
            Marshal.ZeroFreeGlobalAllocUnicode(unmanagedString);
        }
    }
}
"@

$TypeParams = @{
    MemberName = 'ConvertToUnsecureString'
    MemberType = 'ScriptMethod'
    TypeName   = 'System.Security.SecureString'
    Value      = {
        [ExtensionMethods]::ConvertToUnsecureString($this)
    }
}

Add-Type -CompilerParameters $CompilerParams -TypeDefinition $TypeDefinition
Update-TypeData @TypeParams

SecureStringToGlobalAllocUnicode copies a SecureString into unmanaged memory, and PtrToStringUni holds a copy of that unmanaged string until the pointer to it is freed by ZeroFreeGlobalAllocUnicode.

It seems relatively secure, but I'm curious what someone more versed in C# thinks.