This is what I use to have scrips run as admin by default:
Powershell.exe -Command "& {Start-Process PowerShell.exe -Verb RunAs -ArgumentList '-File """%1"""'}"
You'll need to paste that into regedit as the default value for:
HKEY_CLASSES_ROOT\Microsoft.PowerShellScript.1\Shell\Open\Command
Or here's a script that will do it for you:
$hive = [Microsoft.Win32.RegistryKey]::OpenBaseKey('ClassesRoot', 'Default')
$key = $hive.CreateSubKey('Microsoft.PowerShellScript.1\Shell\Open\Command')
$key.SetValue($null, 'Powershell.exe -Command "& {Start-Process PowerShell.exe -Verb RunAs -ArgumentList ''-File """%1"""''}"')