Stealing mass driver packets is cool but I don't think gameplay would benefit considerably or that it would even be a smart thing to do in game. If you want to steal an enemies packets you're going to have to go into hostile territory with a freighter and warship protection for maybe 300 tons of minerals every 5 days. And there's the question, if someone is stealing your mineral packets, why not just turn the mass driver off and wait for them to leave? It's what the player would do if a hostile alien was stealing their minerals. Then you have to have the ship continually place itself between the two bodies to steal the packets, does this take fuel? What if the ship isn't fast enough to get between the two planets when they change their position? It opens up continual cans of worms for something that most players (I think) won't use.
TMaekler's suggestion about ship module mass drivers is very interesting, because that would create a some good gameplay decisions. It could be a massive ship module that's meant to be stuck on space stations. Do you set up a more automated empire that uses a lot of mass drivers and requires a lot of space stations but leaves a ton of breadcrumbs for aliens to follow, or do you use less conspicuous freighters instead? A logistics train based on ship based mass drivers would also have the disadvantage of being crippled if a single link is broken. You could even mix them, using ship based mass drivers in a dead end sector and freighters on the frontier.
You tend to see pretty long trails of mass driver packets already in flight in reasonably big systems, so you could potentially fly down the path of packets and get quite a lot of stuff. In terms of catching them, it kinda seems like it would just amount to intercepting them, which fleets already do when joining task group formations. That part is by all appearances easy. I don't really see the can of worms you speak of.
I could grasp the concept of it not garnering a big gameplay benefit, piracy in general is totally dependent on being able to disappear off sensors and hide. On the other hand, I think this would mainly apply to relatively unguarded systems anyhow. I for one don't always have the resources to build a huge integrated sensor and defense grid to protect every mining operation.
It really depends on how difficult it is to implement. Just using cargo shuttles I think is a good concept at least, since it doesn't require specialized parts (making it viable as an opportunistic thing rather than having to build special ships) and also doesn't demand that said parts be added into the game.
e: Though I do really like the idea of space stations being able to receive mass driver packets, that way you could set up a resource accumulation station in a mining system right next to the jump node. This would potentially serve as both something relatively easy for the enemy (or pirates) to pilfer while also increasing the utility of mass drivers (and their potentially stealable packets) quite a lot since its way more fuel effecient to collect your resources right at the node (rather than making your cargo ship fly potentially quite far into the system and then back again).