Update: I decided that clamping and aligning the mast in the mill was going to be problematic and instead purchased a Bosch 10", 80 tooth, 'multi-material' blade for my mitre saw. It is rated to cut aluminium, plastic, chipboard, MDF, and copper. In contrast to a conventional wood blade, the teeth are raked backwards and every second tooth has bevels rather than being square.
Albeit rather costly for a one off job, this approach produced a beautiful, square, clean cut with zero drama and I would recommend it to anyone wishing to shorten their mast.
The holes in the Gong mast are 5 mm, which is the recommended diameter for an M6 x 1 mm tap into aluminium. So, no drilling required.
Maybe this is all common knowledge for most on this forum but it was definitely learn as you go for me.