My son is about the same age as your daughter, he will be 3 on April 5th and my baby girl was born in May 08. My son unexpectedly potty trained about 2 months after my daughter was born, it was some time in July. We have kept him in cloth diapers at night up until we moved about 1 1/2 months ago. Right now he refuses to wear any clothes what so ever unless we are leaving the house, we put a diaper and pjs on him at night and we wake up with him stripped down to nothing. I do think the naked time has helped because he doesn't need "help" going to the potty so he will get up and go on his own. I really think the diaper is an enabler...they know they have pottied in it for over 2 years and their sibbling's are still aloud to potty in them so why not? I would really suggest leaving her in panties at night, try it out, the worst you could have is a couple extra loads of laundry. Make sure that she goes to the potty as soon as she wakes up. If my son does happen to keep his diaper on, when he wakes up I tell him he better not pee in his diaper, he better RUN to the potty :o)
As for mattress pads, if your using a crib or toddler bed still the mattress has that protection on it so it really isn't a big deal. If she is sleeping with you or, on a big mattress i would suggest on of those nylon covers they use at hotels that go completely over the mattress and zips up at the end, you can justwipe them off vs. an extra load of laundry. Walmart has them for less then $15 depending on size, I have a Cal King and we co-sleep so i was able to find one at Bed Bath and Beyond for about $25 i think.
One more thing while i'm thinking about it, if she uses a potty chair i would put it next to her bed at night so she is able to get up easier and use it. Hope this helps.