We present mesoscopic dissipative particle dynamics-simulations of polyelectrolyte electrophoresis in confined nanogeometries for varying salt concentration and surface slip conditions. Special attention is given to the influence of electroosmotic flow on the migration of the polyelectrolyte. The effective polyelectrolyte mobility is found to depend strongly on the boundary properties, that is, the slip length and the width of the electric double layer. Analytic expressions for the electroosmotic mobility and the total mobility are derived that are in good agreement with the numerical results. The relevant quantity characterizing the effect of slippage is found to be the dimensionless quantity kappa delta(B), where delta(B) is the slip length, and kappa(-1) is an effective electrostatic screening length at the channel boundaries.