Here you go, the system works like a charm but I'm sure there will be some rare edge cases that I'm overlooking.
I'm using a Unit Indexer to link the cargo units to their transports as well as to link a Unit Group to each transport. When a transport takes damage it loops over it's linked Unit Group and deals the same amount of damage to the cargo contained inside.
There's also a hidden passive ability that is used to define which transport (units with cargo abilities) will use this system. Simply add the ability to your Orc Burrow and you should be good to go. Also, make sure to set the TDS_Class variable in the TDS Create Group Preplaced trigger to this ability, otherwise the triggers won't work since they reference this variable throughout.