  We propose a method to identify quasars radiating closest to the Eddington
limit, defining primary and secondary selection criteria in the optical, UV and
X-ray spectral range based on the 4D eigenvector 1 formalism. We then show that
it is possible to derive a redshift-independent estimate of luminosity for
extreme Eddington ratio sources. Using preliminary samples of these sources in
three redshift intervals (as well as two mock samples), we test a range of
cosmological models. Results are consistent with concordance cosmology but the
data are insufficient for deriving strong constraints. Mock samples indicate
that application of the method proposed in this paper using dedicated
observations would allow to set stringent limits on Omega_M and significant
constraints on Omega_Lambda.
