If you're invoking the full flight guarantee, you'll probably:
1) get an op-up and 2) if someone is going to get offloaded, it's probably the staff travel. If a revenue pax gets offloaded, they will get a free ticket and a rebooking and accommodation - however, if a controller or station manager sees his or her flight is at risk of having to offload revenue passengers, they will have already told the check-in staff to ask for volunteers (non-skywards members usually), who will receive various offers of compensation (usually a free ticket, sometimes roundtrip, depending on desperation!).
I think it would only be morally dubious in exceptional cases (e.g. Fukushima meltdown).