Been offered a cheapish Shimano XT middle ring, but will it fit on my Deore FC-M542 cranks?

Shimano Technical do list FC-M542 cranks under the non-series components.

The ring will ‘fit’ as it is the same BCD and is Hollowtech-II, but…

If you have the alloy XT middle, it should fit perfectly and properly on the step on the crankarm.

If you have the composite XT middle, it would require fettling.
I would not attempt this as it may weaken the ring.

So, in summary, you may get it to fit, but could have a potential problem on your bike.

I would not want to ride it with the composite ring.

PaulD