I also had this same question while in the area last weekend. Since I was new to the area, fishing solo in my Outcast Stealth Pro pontoon, I was looking for access points. Someone at Lilly's Landing pointed me in the general area and then I spoke to the folks at the Fall Creek Marina about the ramp. They didn't think there would be any problems with using it the way I was describing. I read this thread prior to visiting and search everywhere for information around the boat ramp (which I know is closed to big boats). There are no signs except your typical "fishing regulations" sign (wish I would have taken a picture). But no signs like: No Trespassing, No parking, Private Property - no signs what so ever - therefore I used the ramp last Sunday and fished that section of the river for 5- 6 hours without problems.
I want to respect the property owners and rules of the area, but after it being suggested by two marinas and lack of signage, I felt it was worth the risk to try it out. If anyone knows anything definitively different, please let me know. I plan to return someday.