Well, it can be done.  It's true that rubber dam, nitrous mask, saliva
ejector--it's a lot of stuff on your face.  But I'm guessing it's mostly
just habit.
    They may be able to give pre-op oral agents--you may wish to ask.  If
there is a dental anesthesiologist in your area you may wish to ask if
he/she ever works with an endodontist.

Steve

It is likely that the person doing the endodontics has elected not to
provide nitrous oxide, for whatever reason(s).  Has nothing to do with
anything getting in the way of performing the root canal..  I use nitrous
oxide all the time for endos.

John