Here is what I have thought of for Steel Mountain. Again, these are thematics driven, not balance driven, but they should be good starting points at the least.
The Unparalleled Prowess ability for Steel Mountain:
Steel Mountain: A steel mountain adept tends to lack some of the hitting power of their two-handed weapon wielding brethren, but you have learned to use your shield as a counter-weight to add additional force to your blows and block attempts to knock your weapon out of your hand, without impairing it from its defensive (or offensive) role. When using a shield, you may designate any one other weapon that you wield in a single hand. You count this weapon as being two-handed whenever it would be beneficial to do so, such as for your damage bonus when attacking with it, power attack, and disarm attempts. You may change which weapon you are so designating once per round on each of your turns as a free action.
And here is my best guess at a Focused Rivalry ability:
If this is overpowered I/we could limit it to functioning against the shields of people in a Steel Mountain stance, or with at least one Steel Mountain maneuver readied, but I don't think that is necessary. If this was a 10 level PrC I wouldn't have done the doubling thing, but as it is I think it is necessary to get such a weak mechanic up to usefulness.Steel Mountain: If they insist on getting all your attacks to hit their shield, then hit it you will! You gain twice your class level as a bonus on all to-hit, opposed checks, and damage rolls involved in a disarm or sunder attempt directed at a shield.