"Execute As" clause with a CLR based procedure

If you have ever tried to write a file to a network share from sql server with a clr procedure using EXECUTE AS '' then you have learned true frustration.
Almost undocumented is that the only domain accounts that will actually be used for CLR pass trhough impersonation are CALLER, OWNER, and SELF.
To use these the dbo itself must be the account being used for access.

"Execute As" clause with a CLR based procedure

The other option is to use windows impersonation inside of your CLR class.