I read that the OpenCL support for ivy-bridge should be present on future even on Linux, so i wonder if there may be other type of impediments on using the Gpu inside the Cpu for viewport and an external nvidia graphic card for the rendering on Blender.

Sure OpenCL isn't on good shape on Blender, seem for bad driver side Amd Radeon while nvidia have few optimizations (so is slow) for OpenCL but seem that for viewport should have no particulary issue with OpenCL.