If this is a serious shoot, it should be handled in a professional manner. Asking for permission before trespassing is always a good idea. Specially given the circumstances, saying one will be there for a handful of minutes and then out, should not be an issue.
Unless the photoshoot is more about the fun factor, and everyone understands what is at stake.
I am a huge advocate for "capture and ask later", though it does not usually work if one is breaking into the Governor's office for a candid photo.
Anyhow, my point is -if it were me, and it were a paid or serious shoot- I'd let someone at the property know "hey, I wanted to jump in, take a few pics of them, and get out" and then spend time admiring the hell out of the property owner, his flowers, building, whatever it might be.
My opinion anyways,