i like it, but i feel it would be a bit overpowered with an attack. Plus i would have to ditch improved evasion at 20th.
how about a lower level ability that grants the ability to take a 10 ft. step? effectively granting the hammerhold knight the ability to advance slowly without provoking attacks of opportunity for the first 2 squares of movement. This is considered a move action, not a 5 ft. step, for the purposes of actions in a round.
simple, you can still attack (although not full attack) and you can get around the battlefield a bit faster than normal, while being defensive.
Edit: im thinking 4th level, as all you gain at that level is the ability to ignore that -2 penalty on attack rolls because of shield encumbrance.