... there is a chance I might visit an old friend on assignment in Perth
I'm in Perth and have toured a fair bit in our southwest regions. July/August is generally the wet months but the rain in Perth is not heavy. You'll find the climate is mediterranean if not like LA. Of course as you head south it gets a little cooler and less predictable but at least at that time of year the crowds won't be there hogging the accomodation.
In as far as riding 100km a day you may find that this won't be enough to get you to the next town. I'd recommend heading south from Bunbury to the wine region at Margaret River, lots to see and do there and nearby. After that you have Augusta then a large gap across to Nannup or Pemberton, both fine towns in an interesting timber region. If you want to venture further Walpole, Denmark then Albany are good choices. There are main roads and backroads, either way I think that you'll find the traffic light once you get away from Perth, Bunbury and the Albany or Bussell Highways. There are restrictions on camping in a lot of shires here but plenty of backpacker hostels and if you like, plenty of hotels.
You might also consider posting on BNA, for more local input.
I am planning on cycling in Australia for 3 weeks in December and am looking for pointers on the best coast, roads, routes. I welcome any tips, suggestions or even better mapmyride.com links that I could use as a starting place to plan my trip.
Also, if you know of any cycling groups that may be good to reach out to, that would be great too.
I am planning on flying into Melbourne to see a friend but am open to flying somewhere else to start.