I have no experience with a metal mast for this magnetic loop. I made an insulating wooden mast.

But could a metal mast be used ?

From literature i concluded that, as long as metal parts in the antenna near field do not resonate, they do not harm the antenna radiation pattern nor the antenna feed point impedance. That is the case, if all these metal parts are not 1/2 lambda long or a multiple of 1/2 lambda long.

A metal loop mast is in very close distance with the antenna. 
(Wikipedia theory) : The mast is in the "reactive" part of the antenna near field. If the mast should react with the antenna radiator, it will be noticed as a wrong antenna impedance at the transmitter side of the feeder.

The insensitivity of this loop antenna to metals near by seems to be confirmed by the photo on the right.
-  A long aluminum household ladder stands was very close to the loop. 
-  A 5m long aluminum mast is only 2m meters away from one side of the loop.

Practice : When the ladder was laid down, the best VSWR frequency, nor the VSWR value of the loop changed. There was no reaction, no change in coupling.

The 3.2m high ladder, and the 5m high mast, did not resonate at 3.5 MHz nor 7.2 MHz. Their 1/2 lambda resonance frequency was far higher. 

The coax feeder and the cable for relay and heating run both up on the wooden mast, in the vertical center line of the loop. These cables have on both ends of the mast Common Mode Chokes, and further away were ferrite clamps installed to prevent common mode cable resonance. These cables also cannot resonate at the frequencies where the loop is transmitting, and also do not influence the loop performance.

I therefore presume that a metal mast is useable, as long as it cannot resonate by its length, INCLUDING the length of the to the mast connected safety ground wire !!