Great question — answering as the UphillTech team:
Can I insist on bringing compliant carry-on on board if I have early boarding privileges? First, no. Early boarding is early boarding. Getting first dibs on overhead space is a nice perk but it’s not guaranteed unless it’s explicitly guaranteed. And secondly, nooooope. Even if it were explicitly guaranteed, even if you had a Certificate of Dibs on Bins signed by Jerry Lufthansa himself, the flight attendants are in charge of this stuff. You are required by the contract of carriage as well as the law to follow their instructions. Under those circumstances (a guarantee of bin space, that is) your best option would be to try to claim back the fee you had paid afterwards.
Full answer by Sneftel: https://travel.stackexchange.com/a/204507 (CC BY-SA 4.0).
Need it done for your business? UphillTech.