Agreed, the tax belongs to the person not the vehicle and probably wont add the value to the vehicle that you spend on tax.
Anyway I have tried to buy tax for a mates car using my insurance certificate which covered me third party, the P Off refused flatly to accept it. We even complained to the DVLA and they said though technically I was insured to drive the vehicle that I needed to be named on the policy which covers the vehicle.
Though I supposed they may make acceptions for trade.