A "slug" is a locomotive unit without its engine. Usually, the engine (otherwise called a "prime mover") is removed and replaced with something of equivalent weight, such as concrete or steel, since weight is very important for tractive effort.
The traction motors are left intact, and the new slug unit must be coupled to a "mother" or control unit. The slug's traction motors operate in tandem with the "mother" unit's to produce the equivalent of two individual units. CSX added cab control features in its 2200 - 2300 series road slug units, so that they could actually be the lead unit, and control the "regular" powered units that each road slug unit was coupled to! The lack of engine noise and relatively cooler temperatures in the slug's cab were the primary reasons for this feature.
The slug's fuel tank could be used for extra fuel storage for the "mother" unit (and other units), provided that a fuel line siphon and pumping system was connected. Remember, the slug unit has absolutely no use for diesel fuel, as it no longer has an engine.
Don't confuse slug units with remote control units, as they are two very different critters! Remote control units are otherwise regular diesel locomotives, but have remote control technology that enable them to be controlled/operated without direct line connections.
Regarding the cowl units being redesigned for freight storage purposes, some form of weight replacement would be needed following the removal of the engine, as tractive effort is the remaining tangible value of the unit after the engine is removed. A lighter unit would have less effective traction. I doubt that the trade-off (extra freight storage space vs. loss of tractive effort) would be thought of as equal value. Additionally, there is the problem of having to disconnect the slug unit and leave it parked somewhere while it is being loaded/unloaded, since it is not going anywhere by itself!
As a history lesson, Baldwin and GM-EMD built combination passenger diesel units with baggage/freight storage compartments. They definitely were not a raving success! Only several demonstrater units were built and sold. No additional sales afterward. That was back when the Railway Express Agency was a big mover of merchandise freight (before FedEx and UPS!), and heavily relied on the railroads to move its less-than-carload freight business.
With today's FedEx, UPS, and everybody else in the freight pickup and delivery business, I can't imagine who would use the space created in a cowl (or any other unit) for freight shipping business.
On the other hand, it is a really cool concept for a model railroad operation!