In addition to the integrated pressure sensor, an integrated air pump in the hub directly connected to the tire and actuated through the brake.
If the sensor detects a pressure drop, the next time you brake, it switches in the pressure generator in the hub which assists in braking and uses the energy to pump up the tire.
This might even be an ideal accessory for cobble racing (e.g. Paris Roubaix): drop tire pressure on the cobbles and reinflate just before getting back on asphalt.
In most cases there is a corner at the junction to the asphalt where you need to brake, so why not use this wasted energy to reinflate the tire.

There will be no additional friction in the hub when the pump element is inactive and the energy loss by having an additional weight is easily saved by improved efficiency on the asphalt and the added bonus that you dont have to stop at the most inconvenient space or time when a service vehicle if away or cant get to you