This is a month after your posting, so you may not be interested anymore, but here's some additional info.  You can lock the autopilot onto the ILS glideslope and localizer by adding a NAV toggle.  As you know, on the 757 there isn't any switch for this, but you can program keystrokes to toggle NAV on and off.  I use Shift+N.  As you begin vectors to the ILS switch off the LNAV button on the autopilot, which disengages the autopilot from the GPS.  Set the ILS freq and course in the Nav 1 radio. Hit your NAV toggle keys as you get close to the localizer and then hit LOC.  Both LOC and the APP lights should come on and the localizer bar on the HSI should be in the correct position left or right, depending on where you are relative to the localizer course.  You should also see the magenta mark indicating you are below the GS.  From that point on the autopilot will turn onto the localizer course when it interceipts it and fly the GS down to touchdown while you adjust airspeed using the IAS settings on the autopilot panel (autothrottle engaged).