The area above a suspended ceiling is not a duct or plenum. It is considered "other air handling space" in the NEC and the rules are not as strict.
This will be regulated by NEC 300.22(C)(2)
from the electrical side
Equipment. Electrical equipment with a metal enclosure, or with a nonmetallic enclosure listed for the use and having adequate fire-resistant and low-smoke-producing characteristics, and associated wiring material suitable for the ambient temperature shall be permitted to be installed in such other space unless prohibited elsewhere in this Code.
It might ultimately be up to the installer to convince the AHJ that the transformer was listed for the application. (particularly the "low smoke" part)
I don't see anything in the white book either way.