Haven't done the Nebo/Eungella road myself but two of my workmates have in 4WDs and it's unsealed all the way passed Eungella dam. I'd recommend dropping down on the Peak Downs Highway from Nebo to Eton, then cutting across to Marian and then up through Marani to Pinnacle if you wanted to go up the pioneer valley. The rise up the front of the coastal range to Eungella is very steep and has steel grids set into the road to help drainage. I won't take my bike up there, and the pub and café at Pinnacle are much better than those up at Eungella anyway.
There is a nice road to ride between Marani in the Pioneer valley and the Bruce Highway north of Mackay, it turns off north about 1 km west of Marani and is called the Marani/Mt Ossa Rd. About 30 to 40 km in all, towards the end there's a right hand turn to stay on the tarmac but it's well marked as the way to Mt Ossa - don't go straight ahead to Cullan. Also the road between Marian and the Bruce Highway is a nice, though short, ride, turn north from Marian over the bridge opposite the new shopping centre.