It would certainly be great if you're the defender to just stuff mines into the phase lane... then you avoid the pesky business of planning your minefield.  I don't think anyone can deny that "in the phase lane" or in the phase lane exit is where we all really want our mines.   But it's open to debate whether this would be oversimplifying the mines.

I definitely don't like the idea of sweepers just clearing the phase lane as they fly through, unless the time it takes to do this is proportional to the mine density.  Otherwise, you simply need to add a sweeper to your fleet to negate an entire minefield.  Or perhaps sweepers can be well less than 100% effective at sweeping -- you'd need multiple passes and/or multiple sweepers to have a reasonable expectation of clearing a path.  Conversely, mines in the lane should have a well less than 100% chance of hitting passing ships, so you'd need a pretty dense field to stop massive fleets.

Also... consider that if you relegate the mines purely to the phase lanes, that an opponent with phase stabilizers could theoretically bypass your mines entirely.  But I guess he'd have to get a ship into the target system first to do it.

There are also other theoretical things to consider that are flexible in the context of the game -- like whether or not it's supposed to be possible to stop and do something like clear (or lay) mines while in a phase jump between planets.  May also want to consider whether friendly mines should affect friendly ships this way -- presumably you'd be slamming into them just like the bad guys, so perhaps a mined phase lane should be destructive to anyone (including traders!).

Not a bad idea, though -- it would eliminate the thousands of rendered mines grinding machines to a halt (just have a single integer displayed somewhere indicating the number of mines in the phase lane), would balance out all the races' mines, and would eliminate the micromanagement of creating effective minefields.