With a lack of transport you will be quite limited in Tassie. The only real public transport options, get you along the bass, cradle, murchison or Lyell highways (i.e the roads that link the 4 cities with cradle, strahan, queenstown, lake stclair etc). I dont think any registered operators travel to scotts peak anymore but I maybe wrong. So before suggesting any trips you really need to get your transport sorted.
I dont know your level of experience so I dont want to list off rivers that maybe beyond your ability. incase you havent already seen check out http://paddletasmania.canoe.org.au/?page_id=15629 for tas river guides and there is also a new guide book Tarkine Trails which describes packrafting in the Tarkine however transport will be a big problem up there. It maybe worth calling Tall timbers at smithton to ask if they do any transport out there because I do know they do tours so maybe willing to drop you off at a bridge.
As far as the Arthur river goes depending on which section you do there can be alot of flat water paddling.