Recent experiments suggest that thermodynamically stable, bicontinuous microemulsions can be achieved in symmetric ternary blends of two homopolymers and a diblock copolymer by formulating alloys with compositions near mean-field isotropic Lifshitz points. We argue that practical application of this design criterion may require use of homopolymers of unequal molecular weights and block copolymers of different architecture. We demonstrate the existence of, and explicitly locate, mean-field isotropic Lifshitz points in ternary blends with homopolymer molecular weight asymmetry and either AB diblock or ABA triblock copolymer architectures. These calculations considerably expand the parameter space for observing bicontinuous microemulsions and allow for more flexibility in tailoring melt rheological properties and solid-state mechanical properties.