Yes, a timer measured to the length of the conveyor, use an RTO (retentive timer) enabled by motor running proof, conveyor empty, or go way out there in abstract space and map the contents and associated measurements of each discrete item with pointers to data maps of each belt contents. Once identified, tracking forward stop, reverse can be done, missing packets detected by photocell. This is really sweet with encoder feedback, but pseudo (measured accel times and decel times of hte objects) code can suffice if the spacing to object ratio is within the resolution of your maps. I have bullet proof package tracking that works with motor starters and one sloppy scalar (V/Hz) VFD doing the spacing control, it is literally tracked in educationally estimated (hand tach calibrated) inches from the "birth eye" a polarized retro sensor before the scale. Once born a new box entry is popped into existence and it position updated in a timed interrupt routine for accuracy. You can even go so far as to display them animated on the HMI and let the operators remove them manually from the system without disturbing the under-achieving BSL or FIFO style of product tracking. Since I have three short belts it made more sense to keep track of up to fifteen packages, but with a long continuout belt, I just drop numbers in a data table at an offset from the pointer which is tied to calculated distance as a real, and then carefully rounded to a float for its index position in the rotating file. You have to have a cleanout mechanism to zero out the end of the belt and move the contents of the files (weight, barcode, whatever else) to the next map input offset. Your units must be optimize-able to fit within the constraints of the file size in a SLC/Micro/PLC-5 to avoid crossin file boundaries, but often, I have found that 256in, meters, feet or some other division, like half-foot resolution.
As long as this map has resolution finer than the size of the packages and gaps it will work. It's much more accurate and efficient than FIFO logic, but hard to explain and follow without a good understanding of how it works.