I am a big proponent of suspension seqatposts…at least on gravel and CX bikes…and hard tails when I used to ride them. I have had a suspension post on those bikes for 20+ years…kinda amazes me more people don’t use them. 
But for a trainer, I don’t think you’ll get much benefit. You would have to have a pretty choppy pedal stroke to get any kind of consistent activation…or have it set so soft you would be bouncing up and down anyway.
I built a rocker plate this summer…still trying to decide if I like it. When seated, it is fine and I can stay seated longer than I normally would before having to stand and get some blood flowing to the nether regions. Standing, however, I lose power and still haven’t quite figured out the right movements to align with the rocker motion (it is opposite of what you experienc eon the road)…so I look like a frog on a skateboard when I get out of the saddle, my power drops and my HR spikes.