On my humble Zoom F1, it not only has the usual "hold" sllder switch, but also an "auto hold" function selectable in the menus. When that is enabled, once you start recording, the device is totally locked including level (which is a push button level so that's easy to implement) and the only way to unlock it is to press the "stop" and "record" buttons at the same time (as I recall it - two buttons, anyway).
Normally locking levels might not be a good thing because if incorrectly set, you'd have to actually stop the recording to change them. But on the SD device, I guess if levels are incorrectly set, it doesn't actually matter.