I think this in an interesting idea. While I understand the general consensus is to try to make SP more attractive and that this is against that, but what if this could be implemented as a replacement for one of the other difficult selling points of SP.

Right now the vesting period needs to be longer than the payout period to prevent voting abuse(voting with an account, powering down and repowering up and voting with another account on the same post). Implementing a fee to power up would combat this same type of abuse as repeated offenders would bleed through their SP faster than they could earn it back through the voting rewards. With the inflation protection of SP there would be some break even holding time, so the net effect would be similiar to the fixed power down schedule. The main difference with replacing the lock in period(3 months) with a fee would be that holders could powerdown instantly at any time without penalty, removing a large negative attribute of steem power and potentially allowing the markets to more efficiently find the price of steem.