Andrew, all I see in 307 is Hargis and Martin. Chaney's front fender becomes visible at 310 in the extra area below the bottom sprocket hole. But 3 seconds have elapsed since frame 255 which aligns with Alrgens 6 so it does not offer any usable info.
The tip of a motorcycle front fender is just becoming visible in z309, which is actually part of the exposure for z308. The whole wheel becomes visible a few frames later.
It is interesting that we never see Jackson in the zfilm. But Jackson and Chaney are riding side by side up Houston as seen in Hughes' film. So can we not assume that in the zfilm Jackson is always out of view to the left side of the frames? In z308 and after we could be seeing Jackson's motorcycle.
In earlier frames, e.g. 193 we see Chaney on the right side at a position that appears to be well ahead of the QM security car an possibly up to the back of the President's car. Is it not possible that Jackson was farther ahead?