If I spot a fleet coming at me I can't seem to right click on it to intercept from a planet. I take it I also can't intercept it once I have left a system because I seem to be "locked in" until I get to the next system. Or is there a tech that will allow me to do one or both things?

Battles can occury on nodes only. 

You can't change direction of flight while moving between nodes.

And you can't station patrol ships in space either. It would be nice if there was something you could do when you send ships towards an enemy area and you see some coming the other way to bomb your own world back to the stone age. At the moment you just wave at them in flight, wait to get to their world then turn around and head back home!

If you could change movement vector mid-flight, it would be literally impossible to catch that fleet.

The closest you get to "open space" is the special nodes. Until free movement becomes commonplace (which, granted, is quite early) you can station ships at special nodes or low-planet systems that act as bottlenecks, if you happen to spot a single-string path between two areas.

Probes and scout ships with high vision radius are your best way of anticipating where ships are going to attack, so you don't send them off into the fog only to pass an enemy.