Retargetable C compilers are key tools for efficient architecture exploration for embedded processors. In this paper we describe a novel approach to retargetable compilation based on LISA, an industrial processor modeling language for efficient ASIP design. In order to circumvent the well-known trade-off between flexibility and code quality in retargetable compilation, we propose a user-guided, semi-automatic methodology that in turn builds on a powerful existing C compiler design platform. Our approach allows to include generated C compilers into the ASIP architecture exploration loop at an early stage, thereby allowing for a more efficient design process and avoiding application/architecture mismatches. We present the corresponding methodology and tool suite and provide experimental data for two real-life embedded processors that prove the feasibility of the approach.