Thank you for all the responses everyone! I've been busy at a games event all day so I couldn't get to replies as soon as I'd have liked to.
When I set Ruth on T-pose in SL (I use the latest Firestorm) most of the clipping occurs at the upper back, tricep area of each arm and the love handles. Oddly enough the exact same avatar shape exported from Blender (as a solitary mesh rather than worn) with the shirt fitted on directly the shirt had absolutely no problems.
My Avastar version appears to be 1.1.2 on my Blender 2.70a, so I'm not entirely sure all the latest updates would fix issues pertaining to the base female avatar clipping in-world when wearing the shirt, while that same shirt is absolutely fine when put on a mannequin of the exact same shape as well as Blender and in-preview before uploading the mesh.
I do hope to get this resolved, as I've had dozens of ideas to experiment at making base clothing in SL obsolete for years now. Thank you all again for the assistance, hope to hear more from you again soon!