Is this legal or not? I know you can install one above a suspended ceiling but can you if it is a plenum ceiling?

I don't see a problem installing a transformer in a space above the ceiling as discribed in 300.22(C). If however, it is indeed a plenum as discribed in (B) you may have a problem. There are other considerations in Article 450 that need to be addressed also such as accessibility and size of transformer.

It is a 45 KVA transformer and it is vented.
I have never done this before as I was told it would have to be a sealed transformer or in an enclosure. It was explained that the dust and lint that is being sucked into the plenum could build up on the windings of a vented transformer and the heat from the transformer could ignite it. This installation would take place in a clothing store in a mall.