This recent aviation.SE question asks why isn't GPS used as a backup speed indicator. The answers given mention that GPS cannot indicate airspeed. However, there are other methods to indicate airspeed. At the very least, a small anemometer and vane might be deployed in a fashion similar to how a ram air turbine is deployed. Or at the very least, deploy the ram air turbine and measure the RPM or voltage.
Another creative method might be to release a small red sponge ball, and have a camera/sensor mounted in the top of the tail (where the A380 already has a camera) determine the vector that it flys away. Image processing for this type of work has been available for years already.
Of course, these solutions would only be deployed when necessary. I wager that using the ram air turbine to determine the airspeed could be done in software with no added weight or mechanical components added to the vehicle. Assuming that the vehicle already has a camera in the tail (such as the A380), adding the ability to deploy a small sponge ball would be less of a retrofit than adding cellular telephone service.