I did read those posts from BOL, and they seem to support my findings that you cannot solve my problem with the EXECUTE AS. Please consider the following scenario:

LOGIN1 is SA

LOGIN2 is not SA and should not have SA rights

After logging in as LOGIN1

CREATE PROC TEMP AS

EXECUTE AS LOGIN = 'LOGIN1'

--Code requiring SA Access

REVERT

After logging in as LOGIN2:

EXEC TEMP

Yields this error:

Cannot execute as the server principal because the principal "LOGIN1" does not exist, this type of principal cannot be impersonated, or you do not have permission.

To overcome this error, I understand that I must give LOGIN2 impersonation access to LOGIN1, and if I do that I've opened up an security hole for LOGIN2 to have unfettered SA access through impersonation--which is the very problem I'm trying to avoid in this exercise.