Give your UU a unique promotion, say PROMOTION_DUMMY, which states that "it can earn unique promotions" (for example). PROMOTION_DUMMY acts as a 'flag' or 'marker' (where it does nothing combat-wise), but can also have some effects itself (think in terms of the XML-tags). You then make sure all your unique promotions require a unit to already have PROMOTION_DUMMY, in the same way PROMOTION_SHOCK_2 requires a unit to already have PROMOTION_SHOCK_1 before it can be chosen.
EDIT: Make sure that you don't have the <CannotBeChosen>-tag set to true!