I have been using a Lowepro 450 AW ProTactic for about 5 years and have travelled with it as cabin luggage on many flights - It fits in overhead lockers and you just have to keep it under 10kg for most airlines.
It will carry my MX1+100-400mm+TC all mounted and, as you say,
works best with righthand grip uppermost. It will also carry my second MX1+40-150mm+TC or 300mm unmounted.
Because I do a lot of photography from beaches and in all weathers, often from dawn-til-dusk, I have its built-in waterproof cover out all the time (except on planes). So in effect I have a semi-rigid 'box' with a zipped lid and don't have to worry about putting it down on wet (or dry) sand for example. I carry a tough nylon brush in the car to brush it down after a session.
The 150-400mm+1.25x will fit when it arrives at Casa Robin.
I also have a Lowepro 300 AW II Flipside which I haven't used much yet and never as a flipside. It was the end of a line in the digital camo design and so I got a discount from WEX who are agents for Lowepro, Gitzo and Manfrotto (all brands owned by the same company) and so are able to offer very competitive prices on these brands. I can check for you what fits it if you tell me which Olympus gear
@Box Brownie.
All my various bags are Lowepro - They simply do everything I need them to do and are extremely durable. I even have a soft Lowepro 300 Photo Sport BP AW II lightweight backpack which can be useful because it can carry non-photo stuff as well but it's not designed for the bigger lenses.
A DAY IN THE OFFICE! by
Robin Procter, on Flickr