If you are determined to include Mt Cobbler in your circuit, then it is quite possible to get a vehicle to Lake Cobbler and work a circuit from there.
The road will vary from year to year, but when we went it was doable in a 2WD.
The last rise up to the lake was fairly steep and tested the engine, but the lake outlet was a small navigable pool and we were able to park near the hut.
A circuit from Cobbler would use Muesli Spur and walk the roads in the valleys as well as the tops.
But I would rather repeat some of the high sections between Cobbler and Speculation to at least Buggery rather than try for a bigger loop.
We also easily got into the Upper Howqua in a small 2WD on that trip via Circuit and Bindaree roads, but again - that will vary from year to year.
We came down Helicopter Spur but you may pick up some ideas on my trip report:
http://www.bushwalk.com/forum/viewtopic.php?f=9&t=19353