> I can't find any other place in WebODF core that does a kind of signal wrapping.

All controllers do some kind of signal wrapping. The metadata controller is somewhat special as it only forwards the internal signal, true.

The reason that consumers (and here consumers are implementors of editors, who want to connect their thin UI to the convenience controllers) should not simply subscribe to the OdtDocument are:
* they should have one partner to deal with, the controller, who abstracts the aspect of the metadata setting/getting away.
* such consumers ideally do not deal with OdtDocument too much, too prevent that they accidentally call modification methods as well, bypassing the op mechanism.