That problem isn't solved by DoH, you still need to hardcode the DoH server IP there too if you intend to bypass the network level settings. So a proprietary system would be no different.
All the major appliance vendors (Google, Amazon) already have huge fixed IP ranges to devote to this purpose, which are effectively unblockable because they might be shared with important cloud services.
All the major appliance vendors (Google, Amazon) already have huge fixed IP ranges to devote to this purpose, which are effectively unblockable because they might be shared with important cloud services.