lock LUT_input pins for arbitrary LUT

I am wondering if there is a way to specify the LUT_inputs and lock them for a given LUT. The reason I am doing this: I am creating delay lines using chains of inverter gates. If I allow the optimizer to create the delay lines, then some of the LEs have inputs on port C while others are on port D. This is problematic because, without control, the delay is varied (port C has a longer delay than port D). Thus, I would like to have arbitrary control over each LE with regards to the LUT and the specific LUT input pins used. What do you think?