If you still have to use the high-risk racks, get a beater-bike, one that looks very down-market. It doesn't have to actually be a bad bike, just a good, old one. My beater bike has nuts and bolts instead of quick-release parts.
Lock it with both a U-lock and a decent cable. Lock both wheels to metalwork embedded into the ground.
Remove anything removable, lights, bottles, bags. Put a plastic bag over the saddle.

If the lockup is high risk, you should be concerned about getting jacked whilst trying to unlock your bike. Try using this as your argument.

Regarding removable stuff, you will eventually drop something, usually the most expensive and brittle thing like a front lamp. Minimize the removables. Do you really need a waterbottle? Is the saddle bag in addition to other bags?