The Soviet Ilyushin 86 widebody, which was somewhat contemporary with the Tristar, was also self handling (in fact even more) where it was wished. In this case there was a lower hold-level entrance, with short airstairs, which led to a lower level baggage counter (and crew seats) where bags were taken from pax who had brought them on board. Pax then climbed stairs to the main seating level. It was thus completely self-sufficient, but outside the Soviet Union was difficult for airport terminals to handle pax going right through to the aircraft carrying hold baggage, so the standard arrangements for that prevailed.
Unfortunately the only decent account of the process I have seen has been ruined by Photobucket withdrawing their links to pictures accompanying a description of this.