In the more than 25 years I've been familiar with the timetable there have been no scheduled services from Yeovil Pen Mill onto the Withered Arm. That said, the section between Yeovil and Exeter has always been used by the Western Region/FGW/whatever you want to call it as a diversionary route.
Regarding 'SWT' or rather its predecessor the South Western Division of the Southern Region, all trains through Axminster were SR loco-hauled prior to introduction of 159s (in my day class 50s on trains to/from Waterloo, 33s on Brighton or Portsmouth workings). There were a couple of WR DMU short workings between Exeter and Honiton, which may some years have been extended to/from Axminster, but other than this it has always been a SR/SWT stronghold for scheduled services.