We study semi-supervised estimation of the full-lifetime Mann-Whitney probability under right censoring using finite covariate-only samples from two populations. A model-assisted estimator is constructed by adjusting censored pair comparisons with differences between labeled and auxiliary feature means. Resource weights minimize first-order variance within the working projection class. Under joint independent censoring and regularity conditions, we establish consistency and asymptotic normality through a four-pool influence representation that accounts for censoring distributions estimated from the same labeled records. Consistent variance estimation from all four sample pools yields Wald confidence intervals. The efficiency analysis relates variance reduction to projected comparison signals and auxiliary sample sizes and characterizes the gap to the semiparametric bound. A fixed-horizon extension accommodates half-weighted ties and the terminal mass induced by capping follow-up. Simulations show increasing precision gains with auxiliary sample size under moderate or strong projected signals, with finite-sample losses in some settings with zero or weak projected signals. A five-year Rotterdam breast-cancer comparison illustrates precision gains and variability across label allocations when auxiliary samples are formed by controlled outcome withholding.

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