As a former gate agent, (approx 15 years ago), I would highly recommend against doing this. The agent will most likely not short check your bag, so you better only have carry-ons. I'd also avoid the hidden city thing. They might just come after you, however it's more likely on a higher dollar ticket like say booking a ticket through LHR to DUB and getting off at LHR.

Anyhow, they can terminate you mileage account if they wish. It's in the terms and conditions