Memorandum to Honorable Kenneth a. Lazarus, Asso. Counsel to the President, from Antonin Scalia, Asst Att’Y Gen., Office of Legal Counsel, Re: Applicability of 3 C.F.R. Part 100 to the President and Vice President (Dec. 19, 1974) has been uploaded by Seth Barrett Tillman to SSRN. The memorandum primarily addresses the meaning of "Officer" in 3 CRF 100, but the analysis is based in part on a supporting argument that the use of "officer" in the Constitution excludes the President.
All the theory that fits.