The Kelly interpretation indirectly addresses your question. The petitioner enquires whether time in an aircraft certificated or more than 10 seats, but which is not configured with those seats (eg, cargo) may credit time in that aircraft toward 121 requirements; the Administrator (via assistant chief legal counsel) states that the answer is "no."
https://www.faa.gov/about/office_org...rpretation.pdf
The Kelly interpretation included a wrong reference and unfortunately required additional clarification. Fortunately, the Administrator did clarify that with a memorandum, which does directly address your question regarding installed (configured) seating:
https://www.faa.gov/about/office_org...rpretation.pdf
Accordingly, this memorandum confirms that§ 135.243(a)(1) requires a PIC to hold an airline transport pilot cetiificate for the following passenger-carrying operations conducted in accordance with part 135: (1) operations using a turbojet airplane; (2) operations using an airplane having a passenger-seat configuration, excluding each crewmember seat, of 10 seats or more; and (3) operations using a multiengine airplane in a commuter operation as defined in pati 119 of Chapter 1, ofTitle 14.