This has been discussed on FT in the past (in that case, a child needed their passport for international travel and the father accompanying the child could get it at the international gateway). Opinions were all over the map, but the majority felt that, if there is one ticket, the airline COULD deny boarding for the domestic portion since they are required to verify documents for international travel at the origin. YMMV, but I would always have my passport if there any chance of having to fly out of the country.