Haddock is on Phabricator and has an .arcconfig file. All you need to do is submit a revision like normally, just run 'arc diff' in the Haddock directory. But this will create a separate revision for haddock, right? My changes in haddock are related to revision I am working on. What I would want to do is somehow have my haddock changes uploaded as part of the diff.
Also, since you have push access to the repository, you can always just push your Haddock changes to a branch; for example, commit them to 'wip/my-cool-thing' and push that branch publicly. Then point your utils/haddock to that SHA1, and 'arc diff'. The rest should work just fine when you run 'arc patch' or 'arc diff', or when Harbormaster builds it. Ok, I suppose this will suffice.
Janek