I imagine you have some idea, but I'd be interested in seeing any calculations (even rough ones) of how you expect this POS operation of yours to be profitable in the long run.Short answer: cloaking devices.
Long answer: The Prototype Cloaking Device blueprint is difficult to make copies of. It requires more than the base skills, some research databases, and reports (or datasheets, I can never remember which). Even then, to make a single full run BPC it takes just over 9 days in a copy slot and you need a max run BPC for useful invention.
Once you get a BPC, you try to invent an Improved Cloaking Device or Covert Op Cloaking Device BPC from it which will have 10 runs. Assuming you are successful (about a 50% success rate) it takes another 2.5 or 4.5 days to build all the modules from it at about a cost of 2-3 million ISK per module and requiring extensive manufacturing skills.
Then you can sell them. The Covert Ops cloaks go for about 7-8 million ISK right now, the Improved ones for about 6 million. So rough ballpark estimate of 4 million ISK per unit, or 40 million ISK per invented BPC.
Now I own two Prototype Cloaking Device BPOs and plan to set them up for constant copying so every 18.5 days I get 4 max run BPCs and then invention usually gives me 2 Tech II BPCs for 20 units at 4 million ISK profit each, or 80 million ISK.
Now the POS will cost about 30-60 million ISK per month in fuel so you can see most of the profit from the cloaks will be eaten up but there will be some left over and all the other slots on the mobile labs are still free for other projects like Invulnerability Field invention, Ship invention, and BPO research.
While I could make as much or more ISK by using publicly available laboratories, the wait queues make the process more onerous and less flexible to change than I like. If I need an Osprey BPC for an invention attempt or something, waiting 20 days for the next free slot is not an attractive option. The POS gives me the flexibility to respond to request while still being profitable.
And the work it takes is kind of fun, like a hobby or something.