I actually had good luck gaining permission at a old Victorian house that was up for sale in my town last year. When I drove past it, I noticed that there was a city notice posted on each of the entry doors saying that an inspector determined the house to be either vacant or abandoned - yet there was a fresh "for sale" sign from a local realtor standing in the front yard. I figured if a local realtor was handling the sale, it probably wasn't abandoned. Since the realtor's office was literally right up the road, it was easy enough to pop in and ask about it.
Being a mid-western small town probably makes a huge difference, but the realtor couldn't have been friendlier. I told her I had noticed the fantastic Victorian down the road that was for sale and I was hoping to gain permission from the proper owner to try metal detecting in the yard. Turns out an elderly couple did still own the house, but they no longer lived in it and couldn't keep up with it anymore. She asked me a few questions and took my card, saying she would call the sellers and see if they would allow it. Well, we've all heard that before - I was polite, courteous, and chatted for a little while before leaving, but I assumed I'd never hear back from her again. No worries, it's part of the hobby, right?
Well, imagine my surprise and delight when my phone rang not even 10 minutes down the road. It was the realtor - she had already spoken to the owners, and they were perfectly fine with me detecting the property! She advised me that I had to be quick, though - the house had an offer on it with closing expected in about a week, so I had to get in before the house changed hands and the permission expired. I turned the car right around and spent the rest of the afternoon at the house, and part of the next day, too!
The moral of the story is, don't be afraid to contact your local realtors about specific homes they have in their inventory that you're interested in detecting - even if you don't get a "yes" right away, they are great contacts to have on your side! Plus, they can be great sources of information on local history as well, because they'll often know the backstory for the properties that they're selling.