FAA ATC is required to assign an altitude with the approach clearance when clearing an aircraft on an unpublished route. See FAAO 7110.65 4-8-1 Approach Clearances. This was very technically legal as the MVA was 7,800 from where the aircraft was located to CEGAN. It was erroneous due to the MVA later on the arc rose to 8,700’. The ATCO would have been better to say, “cross CEGAN at or above 10,000’”.
GF