  We discuss a mixed-action approach in which sea quarks are regularised using
non-perturbatively ${\rm O}(a)$ improved Wilson fermions, while a fully-twisted
tmQCD action is used for valence quarks. In this setup, automatic ${\rm O}(a)$
improvement is preserved for valence observables, apart from small residual
${\rm O}(a)$ effects from the sea. A strategy for matching sea and valence is
set up, and carried out for $N_\mathrm{f}=2+1$ CLS ensembles with open boundary
conditions at several simulation points. The scaling of basic light-quark
observables such as the pseudoscalar meson decay constant is studied, as well
as the isospin splitting of pseudoscalar meson masses.
