After testing Reuniclus, I can confidently say that the Offensive Trick Room variant is the most viable in Dream World. I tried other variants such as the Calm Mind and even Trick sets but I hated the speed of Reuniclus. I found myself luring in Pokemon and Genesect and Chandelure who would either U-Turn out into something else or just straight up OHKO me respectively. I switched to the Trick Room version and have never looked back, it's so good and needs a lot more usage.
Reuniclus has a very good niche in Dream World, the fact that it is one of the few sweepers that does not need to rely on a good speed stat to sweep. Trick Room completely switches the momentum of the match around. Many times has an opponent sent their Genesects or Excadrills in on Reuniclus predicting a Psychic, but instead you Trick Room and then proceed to OHKO them afer hazards. Coupled with hazards, Reuniclus is a very effective late game sweeper especally against the most common variations of Sand Teams. Keldeo, Breloom, Deoxys-S, Excadrill, Tyranitar, Amoonguss, Landorus, Chandelure and Skarmory are all either 2HKO'd by Reuniclus, or OHKO'd after hazards support.
Reuniclus performs very well against the top tier threats, but it does need some support to grant some OHKOs. I personally have been running a more Semi-Stall team built around Reuniclus with Specially Defensive Heatran / Specs Keldeo / Physically Defensive Amoongus / Physically Defensive Skarmory / Scarf Genesect and it's been working really well. Skarmory / Heatran usually always get up Spikes and Stealth Rock in a match, and Keldeo is there to take out the spinners, as Tentacruel and Starmie cannot stay in against Specs Keldeo. Once hazards are up, let something die and send in Reuniclus and it's usually good game there. Nothing really wants to switch in to Reuniclus when hazards are up, perfect coverage means that nothing is safe from Reuniclus. This was helpful against stall teams, with Keldeo supporting Reuniclus they were no trouble. Even higher up the ladder where stall teams were more common, Reuniclus still pulled it's weight. It eats hazards and ignores damage from status, weather and Life Orb. Even the most common spinners in Dream World, namely Excadrill, Tentacruel and Starmie cannot switch in to Reuniclus.
Overall I'm really quite surprised at the lack of Reuniclus usage. On the ladder, I maybe only saw about 2 or 3 in 70 matches. One thing that I think puts people off using Reuniclus is that it is hard to use successfully. People don't want to sacrifice 2 or 3 slots to provide support for Reuniclus. Calm Mind versions lure in the aforementioned threats who all outspeed and can hit Reuniclus hard. It may be hard to fit Reuniclus in on your team, but with the correct support it can single handedly sweep teams.