I've been tinkering with making the UEF playable against the stock Homeworld 2 races in standard multi Deathmatches for my own pleasure. The fighters need work, but the bigger ships are what I'd call highly playable.
I believe I've got it mostly balanced, slightly biased towards an overpowered UEF (let's be honest, I'm a buntu fan). I tried to make the play style of the UEF reflect the way they roll in BP: ships are costly (time and resources) to to build, and they aren't too survivable in all-out combat (especially against Hiigaran beams and massed Vaygr missile fire), but they have a huge weapons range advantage and they are a lot more hyperspace-maneuverable than HW2 ships. In other words, they're good for hit-and-run and standoff-distance combat.
If anyone would be interested to try it out, lemme know.
MatthTheGeek, I've also made a few small additions to the HODs, such as giving Custos, Karuna, and Narayana repairpoints and more comprehensive dockpaths. If you'd like to check out my work, feel free to ask.