Someone has done it.
First thing to do is change the autopilot_available=0 to autopilot_available=1 in the aircraft.cfg.
[autopilot]
autopilot_available= 0
flight_director_available= 0
default_vertical_speed= 700.0
autothrottle_available= 0
pitch_takeoff_ga=8.0
Second thing is to replace the SL30 with the default bendix king autopilot unit in the rxp530 panel.cfg (the last line in the following excerpt):
[VCockpit01]
background_color=0,0,0
size_mm=1024,1024
visible=1
pixel_size=1024,1024
texture=$pan1
gauge00=rxpGNS!GMA340, 4,3, 507,109
gauge01=rxpGNS!GTX327, 4,588, 507,128
gauge02=rxpGNS!GNS530, 4,112, 507,369
//gauge00=Bendix_King_Radio!Bendix-King Radio Audio, 4,3, 507,109
//gauge01=Bendix_King_Radio!Bendix-King Radio Xpndr, 4,588, 507,128
//gauge02=fs9gps!gps_500, 4,112, 507,369
gauge03=..\panel\SierraGauges!p2002_Clock, 550,0,354,114
gauge04=..\panel\SierraGauges!p2002_Hobbs_Meter, 550,122,280,50
gauge05=..\panel\SierraGauges!p2002_EngMgmt, 1021,0,1,1
gauge06=..\panel\SierraGauges!p2002_Rain, 1022,0,1,1
gauge07=Bendix_King_Radio!Bendix-King Radio AP, 4,481, 507,109
In theory that should work, I haven't tested it.