We consider the problem of selective imaging extended reflectors in waveguides using the response matrix of the scattered field obtained with an active array. Selective imaging amounts to being able to focus at the edges of a reflector, which typically give rise to weaker echoes than those coming from its main body. To this end, we propose a selective imaging method that uses projections on low-rank subspaces of a weighted modal projection of the array response matrix, $\widehat{\mathbb{P}}(ω)$. We analyze theoretically our imaging method for a simplified model problem where the scatterer is a vertical one-dimensional perfect reflector. In this case, we show that the rank of $\widehat{\mathbb{P}}(ω)$ equals the size of the reflector divided by the cross-range array resolution. We also derive analytic expressions for the singular vectors of $\widehat{\mathbb{P}}(ω)$ and carry out a detailed theoretical analysis of our selective imaging functional. Our numerical simulations are in very good agreement with the theory and illustrate the robustness of our imaging functional for reflectors of various shapes.