Congratulations, willy, on your noble cause. I wish you all the best in your quest. I have often pondered the possibility of a North-South walking track through Tassie's wilderness areas. I have an old copy of the NWWC's original guide when they first built the Penguin to Pencil Pine track approx 30 years ago. In the description, they said it was their contribution to a future north-south track. Unfortunately, the section from Lake St Clair to Scotts Peak is still missing.
While small in the context of the whole thing, I think the track from the Cradle Visitor Centre to Ronny Creek was a great addition as it links the Penguin to Pencil Pine Track with the Overland Track. Once upon a time, there was talk of a walking track along the river banks at Gunns Plains to avoid roads in that area. I don't think it ever happened though.
Back to the question at hand, the route described by aljscott is certainly the way to go. The road bash from Adamsfield, over Frodshams Pass and down to Scotts Peak is the main detraction from this route for people wanting to minimise road exposure. However, the routes west of Lakes Gordon and Pedder via the Denisons, POWs, etc are massive undertakings. I strongly recommend the following two articles from SUBW as compulsory pre-reading before considering trackless walks in the area:
A Royal ThrashThe Hand that Bashed the BaueraA personal favourite trip of mine was from Mount Cripps to Reynold Falls (not far NW of Cradle Valley). I have a photo of my 2 mates, both 6 foot tall standing in bauera scrub. One of them is standing on the ground. The other looks like he is standing on the first bloke's shoulders. The second bloke is actually standing on a tea tree that was growing horizontally due to the weight of the bauera. This stuff was a good 10 to 12 feet deep. It is not strong enough to support your weight so all you can do is painstakingly thrash your way through. The other trees growing through it such as tea tree, horizontal and scoparia just add to the joy. We made excellent time, covering 1500 metres in only 3 hours. The bauera thickets you will potentially encounter in the SW are far worse and as the SUBW and louphi articles show, a slightly wrong turn can have you thrashing about in it for days.
I can't imagine attempting to run in either horizontal or bauera.
All that having been said, I have heard of plenty of groups doing the King William Range to the Vale of Rasselas trip so it can't be too bad for experienced walkers.