Is it legal to use CT during as Strategic Use to allow a Hidden Deployment unit to enter Suppressive Fire state ? (and obviously appear on table due to entering SF)
From what it appears to me, yes it is legal. CT use of suppression fire states "One Trooper from that player's Army List can activate the Suppressive Fire State (along with their Peripherals, if they are a Controller), following the Activation rules of the State." and it is the last part "following activation rules" which makes it be basically a straight up skill declaration - which is a cancellation clause for both Hidden Deploynent and Camouflage I haven't been able to find any reference to the trooper having to be (physically) on the table.
Further support for this is that in another thread, @ijw suggested that putting a trooper into suppression using a command token will trigger deployables if in their trigger area. Which is kind of a fun interaction in itself. But also supports the idea that it's basically a skill declaration in every way except that it doesn't trigger ARO, so a trooper in Hidden Deployment can probably do it.