Yes, Disney allows backpacks to go on the floor in front of you on the Rock 'n Roller Coaster and Tower of Terror. I know some of the more fearful guests will put a foot through the straps, but on RnR the positive G forces while you are upside down are high enough that they are not going to fall out.
On a side note, Universal on the other hand requires loose articles too be stowed in a locker or with a non-rider on several of their rides. On the ones that do, they provide free lockers for the duration of the ride. Their rides though tend to be more forceful than the ones at Disney or in some cases the ride vehicles are more open so the ride of loose articles flying is greater.