Pasta water usable with lentil pasta?
Yes, you should be fine. Pasta water is used to loosen a condiment, and allow it to be incorporated with the pasta during the final tossing/dressing of the pasta.  The water from traditional pasta will have some starches from the pasta in it, and does sometimes help "thicken" (to a pretty small degree in my experience) a sauce, but that is not typically relied upon.  If your sauce is too thin, just cook it a bit more.  Too thick, add a few tablespoons of pasta water.