Absolute neutrino cross section measurements at the GeV scale are ultimately limited by the knowledge of the initial $ u$ flux. In order to evade such limitation and reach the accuracy that is needed for precision oscillation physics ($sim 1$%), substantial advances in flux measurement techniques are requested. We discuss here the possibility of instrumenting the decay tunnel to identify large-angle positrons and monitor $ u_e$ production from $K^+ rightarrow e^+ u_e pi^0$ decays. This non conventional technique opens up opportunities to measure the $ u_e$ CC cross section at the per cent level in the energy range of interest for DUNE/HK. We discuss the progress in the simulation of the facility (beamline and instrumentation) and the ongoing R&D.
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