At some point, any permutation of the smuggling paradigm can fit, yours works; as well as the guys out of uniform could be Imperials, contracting their smuggling and taking a big cut off the top (which is often how the mob does it today). About the porous borders, if you are doing it, esp getting goods in barter, the fact is somebody already else probably is, and getting cash. As far as cash that the locals are getting, if the Imperials are there in force, then they are spending their drug and prostitute money locally most likely, which is going straight back to the local Mob. If the world is blockaded I would have a tendancy to say the blockade is somehow letting the smugglers in. Like with the Levis, technically they were illegal, a product from the decadent west, but also got a pass for being the uniform of the American Worker.
You're mistaking a customs role with an interdict role.
The navy, during an interdict, simply shoots anyone they catch going up or down. If the violator immediately hove too, he might not get shot... but it's pretty likely he's not getting his ship back. The Navy has NO need for a dirtside presence; they maintain their base off the interdicted world, probably outside the system. Each ship on station has a mate off station - do a 6 week rotation, with 2 weeks home station leave, and a base not more than J2, and use drop tanks to get out if more than J1, and those J4 ships can do it easily.
Two Ship 6 week J1 rotation
Week Ships
1 A on station, B in jump
2 B on station, A jumping
3 B on station, A on leave
4 B on station, A on prep
5 B on station, A in jump
6 A on station, B in jump
7 A on station, B on leave
8 A on station, B on prep
9=1
Fit a jump tank for a two extra weeks + 2x the difference in J-Fuel, and you can have your J4 ship doing 2J2... or, for some designs, 2J3. (Under CT BK5, 2J3 is 20% more fuel than 1J4, 2 extra weeks is 2% more fuel, so +25% is 4/5 the performance on that J4 drive, and 5 extra weeks; 16/5=3r1... And thats before the rules that you can run them below rating to save fuel when not in jump. 8 weeks, however, is about how long you can be out without dumping the LS system... at least if the Type S notes in Sup 7 are not a worst case.
I've used players as think tanks for this kind of stuff, on both sides.
Pretty much, the locals can be cut off pretty easily unless someone off world wants to break them out, unless they have a significant fleet before the interdiction starts. After that, the locals have to kill the interdiction with ground based attacks before they can get anything off world. If they don't have meson guns, anything they can use to do so can be targeted from orbit.
A careful smuggler can get in and out..., but quite likely, not without being seen. The question is one of how serious the interdict is, and how hard the smugglers want to push it.
TANSIS!