One option that might help at least a little bit with your situation, though it won't completely address your request:
Instead of using the cloud sync feature across all your devices, just use that for moving the plan from your Windows machine to one of your devices that you plan to use in the plane during the flight.
Once you're in the plane and the devices you'll use during the flight are all on the same in-plane wifi network, use the real-time flight plan sharing feature:
- On the device that has the flight plan: Flight Plan > More Options > Share with local devices..., then make sure "Auto-share changes" is selected, and click "Share Plan"
- On the other device(s), watch for the alert to pop up that a plan is being shared, and accept the sharing request
The last version of a shared flight plan will stay in memory through an app shutdown/restart, but it's not saved in a folder on the device so there's nothing to delete. The next time you share a plan in real-time, it'll overwrite what was there. Or you can click the "clear" button to clear it out whenever you want to.