I was going to respond to Ewan's excellent analysis but Jim's beaten me to pointing out the CoMC is not really about punishment or rehabilitation.

How about 'in orbit' not around a world or a system that's used but an empty one that, theoretically no one knows about?  Out in the Oort cloud?  This also helps with your need for a system that's NOT (apparently) used or useful but to which the PCs might have to go.  The logic (?) being that real estate in systems with any traffic where they might otherwise be 'buried' is often being discovered, explored, expanded into and so on.

I quite agree with Ewan that this probably isn't cost effective as a normal way of going about things but of course Traveller isn't generally about the 'normal' but special cases.  So.  We want prisoners, of whom there's probably only a handful in the Third Imperium; must be 'disappeared' without being killed; aren't being expected to be rehabilitated etc.  Not sure I've got a good idea for this but (high level) political or (powerful) psionics might fit the bill.

Thus the PCs:

- as low level (scouts/merchants/etc) hear rumours about an oddity in a system they decide to check out.  (Sensor blips?) [1]

- or as higher level (agents of one faction or another?) have to go retrieve one of the prisoners for Reasons (or add a new prisoner/gang to the next ELB in the chain). [2]

- or as very high level (nobles) want to retrieve the man in the iron casket to put on the throne.

In short, yes, maybe this isn't the most obvious/cheap solution but for high adventure or "rule of cool"... why not!?!

tc

[1]  Or how about as another possibility for surviving a misJump?

The PCs end up in said system with no obvious way of refuelling but having discovered the secret of the system once they're there and close up may be able to jury rig some means of survival/escape from the system using tech they find in the set up.  Of course, whether such cannibalization (in extremis, literally?!?) means the current inhabitants of the ELBs can survive is a good question.

[2] This doesn't even have to be part of the "official" (if highly secret, black) process.  Said agents know about the set-up and decide to add a prisoner they want disappearing for their own reasons.