In software product line (SPL) engineering, feature models are the de facto standard for modeling variability. A user can derive products out of a base model by selecting features of interest. Doing it automatically, however, requires a realization model, which is a description of how a base model should be modified when a given feature is selected/unselected. A realization model then necessarily depends on the base metamodel, asking for ad hoc solutions that have flourished in recent years. In this paper, we propose Greal, a generic solution to this problem in the form of (1) a generic declarative realization language that can be automatically composed with one or more base metamodels to yield a domain-specific realization language and (2) a product derivation algorithm applying a realization model to a base model and a resolved model to yield a derived product. We describe how, on top of Greal, we specialized a realization language to support both positive and negative variability, fit the syntax and semantics of the targeted language (BPMN) and take into account modeling practices at Airbus. We report on lessons learned of applying this approach on Program Development Plans based on business process models and discuss open problems.