How to Signal SmartApps -> Custom Devices Handlers?

I would like to signal my custom Z-wave Device Handler when a companion SmartApp finds something buggy (namely, some event sent by the Device was never registered by the ST cloud, getting lost in transit).
I know how to send messages from Device Handlers to SmartApps (after the SmartApp has registered to this specific Device), but I could not find the reverse : state.xxx variables seem to be either local to the SmartApp or local to the Device Handler.
I can probably declare a dummy capability for my Device (a simple temperature sensor) that does NOT exist (such as a dummy switch capability), and then use a command associated to this dummy capability for signaling.
But this seems a bit awkward and convoluted, and I was wondering if there was not some simpler way.

Any suggestion ?