I have done something similar...
There's noting magical about the direction of the load on the lift. The arms are different because a typical vehicle, properly balanced on the lift will be situated so that you need a short arm in a certain position and then a long arm in a certain position.
If you get the car's center of gravity in the right place with the car in backwards, (following your lift instructions) I expect that you will find the arms inadequate and you may have to swap them because the light rear end will be sticking far out the front. This being said.... I doubt that you will actually get more room for your doors. I actually expect that you will have LESS room for the doors to open if you have the car in backwards and properly balanced. (Unless you have suicide doors...)
I had to do something similar to change an oil pan in my car. I had the car in the normal direction and then rolled a cherry picker in from the side to support the engine, disconnected the motor mounts, and then dropped the K Member slightly to get the job done. This moves the load dramatically - moreso than strapping can deal with. Before I attached the cherry picker, I lowered the lift so the rear suspension contacted some jackstands. (So, I had the car on the lift pads AND the rear of the car on jack stands before I attached the engine and disconnected that weight from the car.) When the burden of the engine weight was disconnedted from the suspension, it shifted back to the jack stands and didn't put additional stress on the lift. It worked really well.