Have you seen the trigger channels for Things?
This is an example, where the Velbus names for the channels are read into openHAB first finds and imports the module.
You should even get any virtual buttons in this section.
You’d need to use a rule of some kind.
Trigger would be the button state, either PRESSED LONG_PRESS or RELEASED
Condition the rule by the state of your Outlet device.
Send the opposite command (unless Toggle is a supported command.
As a parallel thought.
Create a rule where the trigger is a state change of your outlet device.
If ON, set the Feedback LED of the Velbus device to SET_LED
If OFF set the Feedback LED of the Velbus device to CLEAR_LED
Plenty of other options