They should fall under part 125.
If you read the chart in the previous post ppt, it shows that large aircraft without common carriage or scheduled service will fall under 125.
Also, if you read 125.1, it reads:
(a) Except as provided in paragraphs (b), (c) and (d) of this section, this part prescribes rules governing the operations of U.S.-registered civil airplanes which have a seating configuration of 20 or more passengers or a maximum payload capacity of 6,000 pounds or more when common carriage is not involved.
(b) The rules of this part do not apply to the operations of airplanes specified in paragraph (a) of this section, when—
(1) They are required to be operated under part 121, 129, 135, or 137 of this chapter;
(2) They have been issued restricted, limited, or provisional airworthiness certificates, special flight permits, or experimental certificates;
(3) They are being operated by a part 125 certificate holder without carrying passengers or cargo under part 91 for training, ferrying, positioning, or maintenance purposes
(4)...
(5)...
(6)...
(7)...
Hope that helps.