Just ask. What's the problem?
Here's a script: "Hello, Mr. Homeowner. I see that you've been trying to sell your property, but it's been taking awhile. And I imagine that writing the check for the mortgage every month on that empty house hurts a bit. I've got a proposal that I think would help you. I'd like to rent your home on a month-to-month basis. That way, you'd be getting rent every month, instead of just having an empty, vacant house. Further, you'd have someone in the house, taking care of it, keeping it clean, having it be occupied, not vacant. And when someone wants to look at your house to buy, what they'd see is an attractive, occupied home. And--ask your agent--that is much more appealing than a vacant home."
I actually know someone who does that full-time. He's a friend of my son, and does it with upscale properties in Florida. The kid's 21 years old and living in multi-million dollar mansions. In his case, he works with several real estate agents who know the benefits of an occupied and well-maintained home over one that's vacant. That'd probably be the best way to do it in your case, too. The script for a Realtor would be similar:
"Hello, Ms. Realtor. Homeowner. I see that you have a number of vacant properties you've been trying to sell for a while. And I imagine that your clients aren't happy about writing the checks for the mortgages every month on their empty houses. And I don't have to tell you that a clean, occupied home is more attractive to buyers than a vacant one. I've got a proposal that I think would help your clients and help you sell those homes, without any cost or effort on your part. I'd like to rent one of your listed vacant homes on a month-to-month basis. That way, you client would be getting monthly rental income, instead of nothing. And you'd have a house that would be clean and occupied. And when someone wants to look at your listing to buy, what they'd see is an attractive, occupied home. Which of your listings do you think this would work best for?"
Hope that helps.