Grace got back to me. The only reason they do not spec Vycor window & door flashing for below grade applications is it has not been tested for hydrostatic pressure. Other than that it should perform just as well as their below grade membrane products. Since my crack is hairline, I do not have concern about bridging strength and because I will have gravel right down to the French drain, there will be zero hydrostatic pressure. So instead of a $130 roll of Mel-Rol, I can get a $30 roll of Vycor Plus from Home Depot.