I hadn't thought of that. Stared at this for a few minutes and couldn't figure out what it should be. The other thought that occurs is that perhaps this would be what is required. (I am of course guessing with nothing to back it up
:SETVAR MyDLLPath "C:\TEMP\MyDLL.dll"
CREATE ASSEMBLY [MyAssembly]
AUTHORIZATION [dbo]
FROM '[$(MyDLLPath)]'
WITH PERMISSION_SET = EXTERNAL_ACCESS;