Who would I contact within Smogon to suggest an Items clause like that?
No one specifically.
You go on to the boards and make your case for why Item Clause should be instituted, preferrably with well-thought out arguments and extensive research like the kind other high-ranking board members bring up to discuss Pokemon movesets. I wager that after that, your consideration will either be discussed thoroughly, or you'll be pointed in the direction of OTHER discussion that explains why they choose to ignore Item Clause, effectively shutting down your argument (at least for the time being).
Smogon is not one guy, or a cabal of shadowy players out to "ruin" Pokemon for everyone. They're a community that makes recommendations for how the metagame can be optimally enjoyed, and the sooner more people learn about this, the sooner we can stop with stupid, baseless whining against the metagame and groups like Smogon.
NOTE: I don't play competitively so I could care less if people play by Smogon's recommendations.
Chaos has it correct. Smogon rules are created and maintained by the community not by one or two people. If you want something to change bring it up as a proper discussion there. Not just a whine and complain fest. Have facts and be prepared to defend them tooth and nail, otherwise it is not worth talking about because it will get shut down very quickly.
The reason you tend to see the same of a specific item is that it's just really good and/or the most effective for that Pokemon's intended role. There are just very few items as good as Leftovers, Focus Sash, etc.
"Murder of the living is tragic, but murder of the idea is unforgivable." - Janus, speaker of the synod