The example you gave is still optional to say yes you have plans or no you don't. An invitation is not a summons. No one asks if you have plans for a specific day or time without a reason. Knowing this, just be honest on if you have plans or not -- even if the plan is to fluff your pillows and stay in bed all day. You sound like you just can't stand the idea that you might miss out on something better. Even if you don't have plans, here's an idea, just tell the person that you don't want to go dumpster diving. Problem solved. If you tell the truth, you'll never get caught in a lie.
If the boss is asking because he needs you to work on an important project that needs to be done at a certain time, they don't usually ask if you have plans, they just tell you they have a project that needs to get done.
"That's not a fair way of asking that allows an employee to speak freely as to what they want to do with their lunchtime." Sure it is, polish up the old spine and say no thank you. Just because you choose not to say no, doesn't mean you don't have the choice.