Whoever said tractor beams, Elite tries to be somewhat hard sci-fi, so unlikely. But what about...
Tow Cable: Stick it on a disabled ship (or mostly-intact wreck) and haul it back for rewards from its owner or the insurance company that now owns it, for salvage rights in some cases, which might get you a free module (randomly determined), or in rare cases, a repairable hull (that requires an extensive rebuild almost as expensive as buying a new ship.)
Other fun uses would also abound. Imperial Eagle annoying your Type 10? Use your superior mass to your advantage! Stick a tow cable on it and simply drag it behind you so that your wingmen / fighters can dispatch it with ease. Prank your friends by picking up and hauling their SRV into orbit.
Towing any ship/object would count its mass against your thrusters & FSD (good luck jumping a Cutter back from the Pleiades with anything smaller than a Type-9), for computing speed and jump ranges. Towed object does not have brakes. Do not make sudden stops. Plan accordingly.
Multiscanner: Acts as both a system scanner and planetary detail scanner. Takes up 3 spaces, because you have to pay for the convenience with bulk.
Hacking Limpet: Initiates a reboot/repair sequence on the ship it attaches to. Probably OP, and might need to be failure-prone or have other measures taken to balance it. May take an ungodly amount of time to "program" before launch.
Smoke Bomb Dispenser: Disperses particulate matter that interferes with sensors, making it impossible to get a sensor or weapons lock on whatever is on the other side of the cloud. Griefer or one of those ubiquitous NPC Condas interdicts your freighter again? Smoke bomb! Ninja vanish!
As for the other ideas...
The "slot splitter" would probably work best if 1 out of every 3 "spaces" was wasted by the rack itself... so a size 3 would only be able to fit 2 size-1 modules, and a size 6 would be able to fit 4 size-1s, 2 size-2s, or any combination that added up to 4. Makes it a bit of a tradeoff that way.