Thought promoting threads are excellent. I've seen some amazing things thrashed out on various forums over the years.
A couple of things you'll notice with hydraulic systems. The actuators aren't that fast to move. A pneumatic cylinder will move faster, but obviously for a given size and system pressure a hydraulic cylinder will exert more force.
I still think you'll be having a lot of trouble basing this on a syncro box.
As far as I can tell, all of the older single clutch sequential gearboxes systems from Ferrari, Lamborgini, Aston Martin and Koenigsegg all activate the clutch to accomidate each gear change, both up and down. If you want to work with syncro equipped gears, I'm sure you'll have to do the same.
Koenigsess even added an internal wet brake to their gearboxes so that rather than rely on the syncros to match the next gears speed, on up changes, the internal wet brake was used to do that and the gear change time was about how long it took for the clutch to disengage and the re-engage.https://www.youtube.com/watch?v=1QJlL0zNdMA
How about a cable shift linkage arrangement? That's how typical transverse mid engined cars do it (not all of them) and the throw and feel is very good in my MR2.
If you do want to persist with automated selection by what ever means, your controlling system will want positional feedback. So any hydraulic or pneumatic cylinder would need slide potentiometers or linear transducers.
Another approach, as was hinted at by 105gta, is electric actuators. Servo motor have built in feedback systems. So the controller tells it to move to 'X' degrees and it runs as fast as it can to 'X' degrees and it will stay there untill told to move somewhere else.