I posed this question to a Talmid Chacham, (I don't recall his name). He said that "The Steipler Gaon" (R' Yaakov Yisrael Kanievsky ZTzL - Father of R' Chaim Kanievsky) pointed out (in response to a similar question), that in a case like this we would not say kol kavua k'mechtza al mechtza. This is because the question does not concern in issur (prohibition) related to the chair itelf, but rather to a chiyuv (obligation) of אִישׁ אִמּוֹ וְאָבִיו תִּירָאוּ [You shall each revere his mother and his father]. Since the question is about wheter the usual occupant of the seat, is or is not the father whom the questioner has to revere, and since the seat's usual occupant is פריש (seperated from all the other people who have seats there), we would say כל דפריש מרובא פריש, and assume that any seat's usual occupant is of the majority who are not his father, and thus he would be allowed to sit in any seat which is available to him.
:-)