If it makes sense to travel along the ecliptic, and pirates know that, then the smart individual merchanter will not tdo that.
Haven't had the issue really come up, but I think IMTU piracy only works as follows:
1) Pirate supplied insider info from someone in the port or providing cargo (an employee of the company, not the cargo owner unless he's a 76-Patrons style insurance fraudsman) provides course info.
2) Pirate agents penetrate the ship as passengers or as crew recruited from the starport (long term deep cover pirate ops with the culmination being a big hit with internal help)
3) Pirates have help from the local patrol! (Or *are* the local patrol)
4) Pirates know that a jump to system X or from system X is most economically (fuel and vector wise) obtained by jumping within X volume of space (X being a subset of all the possible 100+ diameter space!). Then he lies quiet in that 100d zone waiting for an inbound or outbound.
5) Pirate smuggles something aboard a ship (or has a starport authority hook something to the hull near the jump drive controls) which will disable the ship and prevent jump.
6) The pirate is in orbit or nearspace, along with other legitemate traffic (this one works in busy or semi-busy systems) disguised as something else (not getting close enough for visual ID) - another legitemate bit of traffic. When the target ship goes outbound, they arrange to be either crossing its path or passing obliquely behind it. Since they have been hanging around for a while and are legitemate (supposedly) they get to close up before they engage, and hope to target engines and fuel early on... this is 'smash-and-grab' work. Maybe they come in 'englobement' style from a number of spots (the main corsair + other ships or small craft) that were *all* masquerading as legitemate traffic.
7) Lurk on ocean bottom where ocean refueling is expected, a la SDB.
8) Lurk in gas giant.
9) Use planetary shadowing if the economical route to the ideal jump point is (for whatever window makes sense) going to pass nearby the celestial object. Then come screaming out behind the target and hope you have more delta-V and can get him before he hits 100D.
If you use stellar shadowing, the 100D limit for stars can be rather large (out past orbit 3 in some cases...), this can mean you have to travel quite a distance inside the system in realspace. Places like that will be pirate-risky.
Piracy (most succesfully) is done by social engineering - overhearing or feeding drinks to talkative crew in a starport bar (the beautiful agent of the pirates...), bogus packages loaded aboard that the captain can't open, or by corrupt local authorities in zones out beyond the Imperium (think Mexican police...). Masquerades as local traffic or smash and grabs from behind the moon or during refueling can work, but percentages are much lower than when you have good detail on flight plans etc and can lay traps (including minefields or dormant missiles, etc) along the projected route. Similarly, men on board make it much more likely.
I wouldn't constrain ships to the ecliptic. Making piracy easy hasn't ever been something I was concerned about - I think piracy and privateering ought to be tough, then only the best survive and it becomes as much an intelligence/surveillance/human factors job as it does a spacefight.