http://hebrewbooks.org/pdfpager.aspx?req=888&pgnum=193
See end of teshuvah.
I’m not sure what you are getting at, but it would seem that B’dieved (the Eitzah of the Chazon Ish) the “Charaidi” should sit next to the woman but insist that the armrest stay down at all times, so that there is a separation and he doesn’t touch her. He also B’davkah points out that sitting with a man would only be when it is not difficult.
Besides, in reality this should all be besides the point. A “Charad L’Dvar Miztva” would make sure they don’t get into such a situation (as opposed to a bus) by buying a first class seat or two coach seats.