If there are indeed a female surplus, then sending Obi to Taronga to breed (with an imported female) would make the most sense. I know Sydney Zoo had interest in holding both Common and Pygmy Hippos initially so perhaps they may be interested in coming on board as a non breeding holder to support the regional breeding program.
In the past forty or so years we've pretty much only had one facility breeding at a time so the region will ultimately have to get additional regional holders on board to support the program if they wish to have multiple facilities breeding this species (which is in the program's best interests).
I don't foresee this being an issue however considering they're a relatively easy species to hold and would do wonders popularity wise for the smaller regional zoos.
Even one breeding pair of Pygmy hippopotamus could readily supply any upcoming holders, factoring in the surplus we already have in the region (not all five Pygmy hippopotamus are going to be paired with new imports).
Sydney Zoo would be ideal for Pygmy hippopotamus; while it wouldn’t be surprising to see Altina or Mogo acquire them either. They’re a popular species with the general public, with Moo Deng raising awareness/public interest internationally. This was very noticeable when I visited Taronga Zoo, with a few people in the sizeable crowd around the exhibit referencing Moo Deng. Hype fades, but the species became popular for a reason - as did other species like Fiona, Knut etc.
The requirements of Common hippopotamus (filtration infrastructure, space etc) limit their appeal to holders, but I would never be surprised if Australia Zoo gave consideration to them.