Soil vent pipe alternatives

My house only has a downstairs toilet (yes, I know, but we're used to it). The pan is located on an outside wall and exits through the floor and into the sewer from there (no idea of the depth)

There's an asbestos concrete soil vent pipe on the other side of the wall, in what used to be a carport to the side of the house. The vent pipe extends to the roofline of the house. Naturally, the vent pipe is the standard 110mm diameter (or whatever the standard was when the house was built (mid to late 60s)

I've converted the carport into a garage, but the vent pipe is using up almost 6" of width, preventing me parking the car close to the wall. This wasn't a problem while I had a Westfield parked in there, but now I have a Vantage, it's a bit of a squeeze, so I could do with having the car as close to the wall as possible.

An AAV won't help, unless it can be made to be flush with the ground (which I doubt, as I understand that the AAV needs to be above the level of any other appliances - i.e the wash basin)

So - is it possible / permissible to 'convert' the 110mm round pipe to a flat rectangular section for the first four feet or so above ground, then revert back to round and top it with an AAV? A flat section of (say) 20cm x 5cm would have the same cross-sectional area, so the air flow rate would be identical, and it would give me some valuable width back.

So long as it's above any soil connection, it's only purpose is to relieve pressure in the system. Unless the cross-sectional area is absolutely tiny, it'll do that just fine, whatever shape it is. You don't really need to revert it back to round at the top, BUT it's inadvisable to top it with an AAV unless there's an unrestricted vent pipe somewhere else on the system (preferably at the head... you need at least one vent pipe that can vent out as well as draw air in, to relieve pressure of any gasses building up in the sewer. It should terminate above eaves level with a bird-proof cage or vent terminal (with an open pipe, you obviously have to terminate it high enough that the smell isn't a problem, and at least a metre above any nearby window opening).