I am not aware of the term 'plenum rated' ever being applied to LFMC.
Nor is that rating necessarily relevant. You have not described a plenum, you have described a suspended ceiling. Most suspended ceilings are not plenums, and the use of flexible cords is still proscribed.
We recently had here a lively discussion regarding the use of flexible cords within a raceway. That, perhaps, is what you are describing here. It's certainly the only way I can see to somehow transition from LFMC to flexible cord, unless you want to mount a box directly to the ceiling grid.
Naturally, you have to attach such a box to the grid- yet you are not allowed to 'support' the box with the grid. go figure.
I can think of all manner of ways to do what you propose to do, yet I cannot find any that is absoluttely compliant with the latest code editions- even though all manner of listed gizmos are made for mounting things to the grid! For example, the 'cord in the pipe' thread led to some lively discussion.
