GDC says
machining of "raw material" parts is allowed. I'll grant that the FTC stuff isn't raw material, but it'd be incredibly perverse to disallow machining FTC framing, so I say go ahead and bend the heck out of it.
Try to get some welding in too. I tossed it in to see if they'd balk, and they surprised me, so now I want to see some nicely welded minibot frames.