This is just a WAG but.... I think the FOs can log SIC on all legs with PAX under FAR 61.51(f) and 135.101, 105, and could log PIC on Part 91 empty legs under 61.51(e).

That being said, would their first regional employer even care if all the piston time was logged as PIC? An eyebrow might get raised if the candidate had logged PIC turbine.

Part 61.51(e)
Logging pilot-in-command flight time states under (1) "a recreational, private, or commercial pilot may log pilot-in-command time only for that flight time during which that person - (i) is the sole manipulator of the controls of an aircraft which the pilot is rated"

Part 61.51(f)
Logging second-in-command flight time states "a person may may log second-in-command time only for that flight time during which that person: (1) is qualified in accordance with the second-in-command requirements of 61.55 of this part, and occupies a crewmember station in an aircraft that requires more than one pilot by the aircraft's type certificate; or (2) holds the appropriate category, class, and instrument rating (if an instrument rating is required for the flight) for the aircraft being flown, and more than one pilot is required under the type certification of the aircraft or the regulations under which the flight is being conducted."

Part 135.101
Second in command required under IFR.

Except as provided in Sec. 135.105, no person may operate an aircraft carrying passengers under IFR unless there is a second in command in the aircraft.

Part 135.105

Exception to second in command requirement: Approval for use of autopilot system.

(a) Except as provided in Secs. 135.99 and 135.111, unless two pilots are required by this chapter for operations under VFR, a person may operate an aircraft without a second in command, if it is equipped with an operative approved autopilot system and the use of that system is authorized by appropriate operations specifications. No certificate holder may use any person, nor may any person serve, as a pilot in command under this section of an aircraft operated in a commuter operation as defined in part 119 of this chapter unless that person has at least 100 hours pilot in command flight time in the make and model of aircraft to be flown and has met all other applicable requirements of this part.
(b) The certificate holder may apply for an amendment of its operations specifications to authorize the use of an autopilot system in place of a second in command.
(c) The Administrator issues an amendment to the operations specifications authorizing the use of an autopilot system, in place of a second in command, if--
(1) The autopilot is capable of operating the aircraft controls to maintain flight and maneuver it about the three axes; and
(2) The certificate holder shows, to the satisfaction of the Administrator, that operations using the autopilot system can be conducted safely and in compliance with this part.