I've already tried to do exactly what you have in mind to a Tortec Ultralight.
It didn't go at all well.
It looks as if when they are made, at the flattened section there is a closely fitting slug of aluminium inside the tube. As such, the flattened section is essentially solid rather than being a tube.
You'd need a hell of a lot of force to flatten a solid piece of aluminium of that diameter, and without the aluminium insert, it would just fold and crack when you attempt to flatten it.
I can't remember how I planned to get around this, but I ended up snapping a cheap drill bit in the rack (probably out of frustration!). I couldn't get it out so I binned it.
I ended up buying purpose made 26" rack off
ebay for peanuts. It's a great bit of kit, although I used the fitting kit from the tortec, as I didn't bin those bits. The fitting kit which came with the ebay rack was rubbish, but the rack itself is low enough that I can use a Carradice and there is enough space underneath it to fit my tent on top of the rack.