Novel formulations of Gibbs-Duhem integrations are proposed to trace phase diagrams for multicomponent fluid mixtures, including generalized bubble-dew lines, whereby the composition of one of the phases is specified, but may not be constant along the integration path. The algorithms presented in this work constitute an extension of a recently proposed pseudo-ensemble framework. Different types of phase equilibrium calculations with open ensembles are cast into a unique formulation, which leads to a simple and flexible integration scheme for stepping along a coexistence line. This approach is applied to trace phase diagrams for systems described by realistic force fields, including hydrocarbon mixtures. A variant of a Gibbs ensemble that permits simulation of bubble-dew point pressures is also implemented.