And if it's a true emergency, remember that you as the substitute pilot get to pick the field, straight into the wind, with a long runway and cavok conditions, if fuel allows. If you land the plane then that's good enough for everybody. It doesn't have to be at the right airport, on schedule.
You'll be on a discrete emergency frequency and I'm pretty sure ATC will find you a nice and friendly type-rated captain to talk you through everything. With a bit of luck you'll have a chase plane next to you to visually guide you to the approach path, and to provide moral support. No speed limits or other restrictions will apply so you can intercept the ILS (or let the A/P intercept the ILS) at 10.000 feet if necessary, while in the approach configuration & at approach speed.
And if you can't see the runway, look for flashing blue lights. The runway will be next to them.
Biggest problem that I would see in a cavok, light wind, manual approach & landing is judging at what height to flare and how much.