@Denis130 I agree, I wouldn't install a grab bar that wasn't securely fastened into the wall. As for a tub mat, those are designed not to slip around, regardless of how much soap is there, aren't they? Or you can get those stick-on non-slip decal thingies.
Since it sounds like you have gotten a few of these "suggestions" and may attract an older demographic of guests, you might think about putting a disclaimer in your listing- "Please note, if you have mobility issues, you should be aware that there is no grab bar in the bathtub and no way of installing one as it is a preformed unit ".
I'm sure thankful that I don't require such things myself. At almost 72, I still go up on ladders to change out fan blades and such.